The Pokémon games have more than their fair share of Guide Dang Its. NOTE: Pokémon Stadium 2 provided information in-game for Generations I & II that couldn't be found elsewhere in those games, providing an exclusive guide to the games... in a different game.
The three legendary golems in Generation III. Each beast occupies its own inconspicuous cave; these caves are scattered throughout Hoenn. However, to even get into the caves, they need to be unlocked. To unlock them, you need to use Dive in a tiny patch of deep water on a route at the end of some very fast currents; simply getting to the spot is a result of either trial and error or pure chance. Once you Dive and get into the cavern, you need to be able to read Braille. With your eyes. (And if you're playing a video game to begin with, you probably can't read Braille.) Though, mercifully, there is a Braille alphabet right outside. On the other hand, you'd need to know Braille to know it was the Braille alphabet. The Braille writing tells you to how to progress - these clues include using Dig on a wall instead of using it to try to leave the cave like you normally would, as well as putting a Wailord and a Relicanth (two relatively hard-to-get Pokémon that most trainers never have in their parties, especially not simultaneously) in specific spots in your party. Then you have to FIND the now-open caves. What's even more annoying, each cave has their own little Braille test before you can even get to the Pokémon; one requires you to stand in place, not touching your Game Boy, for two minutes. After all that, you still have to catch the Pokémon, who all have a catch rate of 3 out of 255.
The instruction manual includes a Braille alphabet, although some versions don't explain why.
In Emerald, the way to activate the golems was slightly altered from its counterparts. For example, the positions of Relicanth and Wailord in the party needed to open the caves were swapped, and the Braille test for each golem was either slightly changed or reworked entirely. For example, one new task was to run counterclockwise around the walls of the cave, instead of the aforementioned waiting two minutes.
Regirock and Registeel aren't even that hard to find. The caves they're located in are in fairly obvious locations, being large rocks with smaller, obviously assembled rocks around them with an archaeologist investigating them. The problem is Regice, since the cave it's located on is on an island on the westernmost water route in Hoenn. The first time you travel through this route, you do so by speeding on a boat, only giving you a brief chance to see the outermost shore of the island, meaning that you don't even get to know there's anything there. Most players would probably only find it by getting Surf and deciding to go thoroughly exploring previously boat-traveled water routes.
In Generation V, you can get Regirock after deciphering an odd but fairly easy puzzle. You may think that you need to solve a puzzle of similar means to find the other two, but no. Registeel is in Black 2 and Regice is in White 2. You need to go to the title screen to your Unova Link settings, manually change the location of the Rock Chamber to either the Ice Chamber or Steel Chamber, and then go capture the mon. Then, you need to trade over the correct key using the Unova Link settings and two DS systems to get the other mon you don't have. MUCH easier than all that malarkey above, but nothing in the game mentions anything about Unova Link, besides the whole flashback thing.
But for those who bought a cheap used cartridge (without an instruction manual), not knowing to cut open the door to the Dotted Hole on Six Island in the Generation I remakes is extremely frustrating because it says "cut" on the door, but in Japanese Braille. Name another door you have to cut open, and not just walk through, after using a key if necessary.
Bagon in Generation III, especially in Ruby and Sapphire. It is found in the deepest room in Meteor Falls, an area of the game you most likely forgot about after going through it unless you're playing Emerald in which case you return there to fight Steven. Oh, and don't forget to bring a Pokemon with Surf and Waterfall, since they are both needed to progress to the other parts of the area. By the way, this new section of Meteor Falls has trainers in it before you can even reach the room. You can find a Dragon Fang in the small, mostly water-filled room where Bagon is, but other than that, there is no other indication that there is anything special in this room. Bagon has a 25% chance of appearing on the ground section of this area with the added possibility of encountering one beyond lv. 30 (the level in which it evolves to Shelgon), but it is well worth it, since it evolves into the pseudo legendary Salamence.
Bagon became slightly easier to find in later generations, but only through the use of the Pokeradar in the Gen IV Games and the Safari Zone in the Gen II remakes. In Gen V, Bagon was available in White Forest in White version and Hidden Grottoes in the sequels. Not to mention a huge amount of points were needed to find it in the Dream World. In Gen VI, it can be found and trained before you even fight the second gym — it's not even out of the way, though it does appear rather infrequently.
Spiritomb. The part about putting the Odd Keystone into the Ruined Tower is intuitive enough, as examining the latter suggests that something could be put in there and hints at what it is, but the other requirement involves interacting with other people in the Underground at least 32 times. There is one trainer on a nearby route who gives you a hint when you talk to him after beating him, though.
The person who gives you your first Odd Keystone does give you some hints on what to do with it... Although he's fairly vague, and nowhere does the game specify exactly how many people you're supposed to talk to while in the Underground.
The resident non-legendary Game Breaker Garchomp. Catch a Gible. Pokédex says it's in Wayward Cave, the cave accessed by cutting down some trees by the Bicycle Path and wandering past the grass there? After a couple hours of wandering (and helping Mira out), you'll probably figure out that there's no Gible there. Now go look for an alternate entrance to the cave, one which is blocked from view by the Bicycle Path running above you. You need a Pokémon with Strength and Flash. Then, go to the basement and catch one, while you complete a semi-difficult bike course, at the end of which you find the Earthquake TM.
Gible also appears in the Generation II remakes in the Safari Zone, but only if you've completed the two prerequisite quests to unlock the ability to combine certain areas, two of which are needed to find Gible. Good luck finding which areas. You still have to wait over a hundred days for the areas to level up and increase the chances of finding one from 0% to 5%.
Alternatively, you could just catch a Gabite at Victory Road, but only in Platinum.
After acquiring the National Pokedex and Poke Radar in Generation IV, or just the latter in Generation VI, someone (Rowan's assistant, the spare Dawn/Lucas, in Gen IV) mentions you can find coveted shiny Pokemon by using the latter. The actual method, called "chaining", takes ten pages or so to explain, but the simple version is: if you KO or catch a Pokemon in a Radar encounter, the Radar will continue to trigger Pokemon; by traveling to the correct patch of shaking grass note identifying this is what takes ten pages without any extra encounters, you can find that species again, and the Radar will continue to trigger encounters with the same species of Pokemon this way (making this also a useful trick for Effort Value training, which requires headhunting certain species of Pokemon). None of this is mentioned beyond "sometimes if you use the Poke Radar, differently-colored Pokemon appear".
There is a Guide Dang It with the Safari Zone itself. When the Warden tells you about objects, he doesn't really mention that you simply have to press "A" in the Safari Zone to place the objects. Thankfully, it's a mild example, but easy to miss if you didn't already know.
Another one, exclusive to Generation III: So it looks like there's a hole in your Hoenn Dex at No. 151. Turns out it's Chimecho. It can only be encountered in one place, the top of Mt. Pyre, which the player will probably only visit when he needs to progress the story. And even when the player is in the area, the grass is out of the way. And even if the player goes to the grass, the chance of encountering a Chimecho is only 1%. So you've got a very rare non-legendary who no one in the game uses or seems to know about hiding in a very out-of-the-way area with no hints that it's there. Well, after going through all that trouble to find this rare Pokémon, is it at least any good? It has stats befitting of a Com Mon.
Patched in Generation IV, where Chimecho's pre-evolved form Chingling is a constant nuisance.
This same patch of grass also contains the occasional Duskull/Shuppet, depending on version, which is otherwise unobtainable without trading.
With the Generation II remakes HeartGold and SoulSilver, we have a few Pokémon of the third and fourth generations hiding in trees. Now, there are a handful of specimens that are found in very specific trees (and every single tree in the game can be headbutted, so we're talking about thousands of trees). One example: Taillow is found at Cherrygrove, the first city you visit in the game, after a pool of water and over a pile of rocks that can be only climbed after you beat all the 16 Gyms. There are four trees, and you need to headbutt the lower left tree. Even then you can still fight other Pokémon.
In Pokémon Black and White, in order to encounter Landorus you need both Tornadus and Thundurus in your party. However, make sure not to forget to bring your own Tornadus/Thundurus, because if the one available on the game you want to catch Landorus on is not yours, then you won't be able to get Landorus. Nowhere is this mentioned, and it can render Landorus Lost Forever.
Munchlax, the baby Snorlax, and the only means of getting Snorlax in Gen IV. It can only be found in Honey Trees, which involves lathering honey on trees, waiting at least 6 hours for a Pokémon to appear (and not more than 24 hours, or the Pokémon will disappear) and then checking the tree, but Munchlax only appear in 4 of the 21 Honey trees. The trees are spread randomly all over the region and you're likely to never come across at least a few. And these four trees aren't the same for everyone - they are randomised and different for each individual save file. And the game gives no indication or hint as to which trees have Munchlax in them, so you're forced to slather all the trees until you find one. Even then, you're STILL at the mercy of the RNG, because each of the four special trees has a 1% chance of attracting a Munchlax. Save Scumming won't help; the Pokémon you'll get from a Honey Tree is determined when you slap on the honey, but you can't tell what it will be until 6 hours later. Went to a tree, saved, and found a Combee? Reset all you want. It's still Combee. (Though this is useful for finding female Combee, which are required to get Vespiquen, another Guide Dang It if you didn't know only female Combee evolve. You probably won't discover this by accident, because almost 90% of Combee are male.) And that's not all! If you add more honey to a tree immediately after failing to get Munchlax from it, it has a bonus 90% chance to select another Pokémon from the regular non-Munchlax group, so even if it's a Munchlax tree it now has a 0.1% chance of giving you what you want. You have to harvest another tree before slathering the first one. Not only is this arbitrary and counterintuitive, the game encourages you to screw your chances of success by asking if you want to add honey right after fighting the tree's Pokémon.
If you have Generation I or II remakes, you can catch a Snorlax there, transfer it (Requires an older DS or two DS) and breed it with a Ditto or something while the Snorlax holds a Full Incense (Or simply breed the Snorlax in the Generation II remakes and transfer the Egg/newlybred Munchlax). Believe it or not, this is the simpler option.
Dialga and Palkia in Platinum. First, you must find the Adamant and Lustrous Orbs, located in an out-of-the-way cavern in Mt. Coronet that is unlikely to be discovered by most players. Then, you must travel to the Spear Pillar. Players of Diamond and Pearl know that there is no post-story reason to return to the Spear Pillar except to play the Azure Flutenote which is unobtainable without cheating. Sure, people might want to go to the Spear Pillar just to see if the portal to the Distortion World is still there, but it's not likely that they'd find the orbs first. Cynthia's grandmother does hint that the orbs can summon their respective dragons, at least, but that still doesn't tell you where the orbs are.
Uxie, Mesprit and Azelf in Black 2 and White 2. When you enter the Cave of Being after beating the game, the three Pokemon will fly off so you can catch them. The problem is, you have no idea where they are, and they turn invisible until you reveal them. Bianca will show up at your house to hint at their locations (and even straight up tell you Azelf is on Route 23; surprisingly nice of her if you consider the other examples on this page), and they only reveal themselves after you stand in ONE very specific spot in the entire overworld. They will not be there prior to your arrival like other legendaries. The most frustrating of these to find is Azelf, who appears in a completely random spot on Route 23; at least Uxie and Mesprit appear in front of the Nacrene museum and in front of the stairs on Celestial Tower's roof respectively, so if you try to access either location you'll run into them eventually (although guessing they're waiting invisibly for you to find them there is still a long stretch).
Bianca (who happens to be visiting your house after triggering the event) clues about where to find the Pokémon are a little roundabout though, such as talking about them being near what they embody (Azelf is Willpower, so near Victory Road, Uxie is Knowledge so Nacrene museum, and Mesprit is Emotion, so the Celestial Tower roof). However, knowing that Bianca tells you these things is a Guide Dang It in itself (or if you happen to speak to her after rematching the Elite Four.)
X and Y have a fair bit of this. The regional Dex is massive, containing a whopping 454 Pokémon. What this means is that nearly every route has at least one rare Pokémon that only shows up there.
Milotic in Generations III and IV. First you should have realized, by some epiphany of intuition, that the long river in Route 119 (in Generation III) or the gigantic lake under Mt. Coronet (in Generation IV) might be hiding more than Carvanha, Barboach and Magikarp. Inspired by this, you should have obviously tried fishing in every single water tile. Because, see, out of the hundreds of water tiles only in six (four in Generation IV) you may encounter anything out of the ordinary at all, and even then only at 35% probability. This means that if you were to try fishing in every single tile you would still end up with squat roughly one time out of five- not that you would know that there is anything special in there to begin with, of course. Now if you're exceptionally lucky you've found some ugly brown fish called Feebas which your Pokédex urges you to ignore and cannot do anything but Splash around. At this point if you have an ounce of Genre Savvy in you you'll go through the painful process of equipping it with Exp. Share and leveling it up, waiting in anticipation until the moment it evolves. Except, it doesn't. Even if you've raised it to Lv. 100. You should then realize that making Feebas evolve is not just a matter of leveling, but also of beauty, a stat you only ever need for Pokémon Contests, and try to feed it blue PokéBlocks (in Generation III) or Poffins (in Generation IV). If you were to feed it PokéBlocks or Poffins that were not of high enough quality, or you were unlucky enough to have reeled up a Feebas that just hates blue PokéBlocks or Poffins for whatever reason, you'll end up with a not quite sufficiently beautiful ugly brown fish which will get you nowhere, and even if it might occur to you that something has gone wrong, you would have to go back and fish up another one. You might think yourself clever for remembering where you got the last one, but alas, (in Generation IV, at least) the tiles where it appears are randomized every day. Now, given that you have acquired this brown fish, and GIVEN that you have managed to feed it beautifying PokéBlocks or Poffins of sufficient quality, the next time you level up your ugly brown fish it should evolve into Milotic (and if you happened to level the Feebas to 100 for whatever reason...well, it sucks to be you).
You can also just catch one and breed it in hopes of getting one that doesn't hate blue PokéBlocks or Poffins...but only if the Feebas you caught is female (unless you have a Ditto, which would be far less likely in Generation III than Generation IV).
The Generation II remakes are no fairer. Firstly, you can only get Feebas through the Pokéwalker in one route, this is also the last route you would normally get, Quiet Cave, which needs a massive 100,000 watts to unlock. Thankfully, Feebas is quite easy to catch in this one. You only need 500 steps on the route to make it appear, it shows up at a reasonable rate (20%). Saw a Chingling? You'd better send another Pokémon over because the game has randomly decided to not let you get a Feebas this time around! And once you caught it you still need to evolve it. But wait again! Poffins don't exist in Johto! How are you going to max out the beauty stat? Turns out that the only way is to let Daisy Groom your Feebas enough times to raise its beauty. But she only grooms your Pokémon between 3pm and 4pm each day and she'll only do one groom per day. So you need to do this multiple times in the week and then level it up for it to evolve into the long-wanted Milotic. Most people either trade the Feebas over to raise beauty by Poffins, or grab one by the GTS.
Provided you found out about Milotic evolving via Beauty stat (which was never stated anywhere), Emerald was somewhat kind to you on this matter, with the inclusion of the Blend Master; a PokéBlock blending opponent who always uses the very best Berries in the game (Spelon, Watmel, Belue, Durin and Pamtre, if you were interested) and always scores perfect hits on the Berry Blender ... but is only unlocked after you beat the Elite Four, and whose prescence is only announced on the television, so if you don't watch the television, odds are you'll miss him. If not, the only way for you to get those ultimate Berries yourself is to tell one of five incredibly bizarre and unintuitive phrases to the Berry Master's wife, and even then you only get one of each. Want to grow some more? They take many, many hours to grow, and require lots of watering. Hard enough? Until Generation V, this was one of the easiest ways to get Milotic. Yeah.
Fortunately, when all is said and done, Generation V has greatly simplified the process: Feebas can be found on any tile in its route, there are special tiles you can fish on but they're marked this time, and they include better chances at Feebas and even a chance at wild Milotic, and (as contest stats are no more) Feebas now evolves by being traded with the Prism Scale, which even mimics Milotic's color scheme (trade-with-item evolutions are still Guide Dang It material in most cases, but they're a lot less annoying than dealing with the contest stats, provided you have someone to trade with). However, (hardcore) gamers still can evolve the Feebas via the old way in this generation, if one trades a Feebas with maxed Beauty into one of these games.
This isn't helpful for veterans who assumed that Feebas evolved the old way ... only to find that contests were abandoned.
The Feebas line is unobtainable in Generation VI (Except through PokéBank), but the Prism Scale is still obtainable via Pickup.
Shedinja. Shedinja cannot be found in the wild and despite appearing in the Pokédex after Nincada and Ninjask and obviously being from the same evolution line, doesn't evolve from anything. To get it, the player must raise a Nincada to Level 20 and let it evolve with at least one empty slot in their party and at least one Poké Ball with them. Having the Poké Ball is pretty likely already, but most players go around with a full party for the entire game as soon as they are able to do so. Shedinja is also never mentioned in any of the games or used by anyone outside of the Battle Tower (where opponents are randomly generated nor are their Pokémon logged in the 'dex after battle, so good luck to you if you forgot the name of that weird brown bug you saw that one time) and, in the Generation II remakes, Bugsy's rematch team.
From Generation IV on, it has to be a regular red-and-white Poké Ball, specifically (not even a Premier Ball will do).
A few Pokémon evolutions in general are Guide Dang It in the more recent generations and especially in the fourth one, with what things like certain stat values, time of day, gender or even location affecting Pokémon evolutions. Want a Glaceon? Well, if you have an Eevee, and happen to level it up near an icy rock on Route 217... Consider that Eevee evolves in seven different ways: Used a Water Stone on it? It evolves into Vaporeon. Got it too happy during the day? It evolves into Espeon. Got it too happy during the night? Yep, it evolves into Umbreon.
Luckily, you can just press B to cancel an evolution, though even this is All There in the Manual... however, this doesn't work if you used an Evolution Stone to evolve it.
Were you hanging on to Eevee in the hopes of snagging an Espeon or Umbreon for your team, and happened to level up while training in either Eterna Forest or on Route 217? Even if this just so happened to be the level in which your Eevee reached max happinessnote although friendship-based evolutions actually occur when the Pokémon reaches 220 happiness as opposed to the maximum (255), the Mossy/Icy Rock evolutions take precedence over the happiness evolutions. Hopefully you realized this during the very brief evolution scene and pressed B in time. But if you either didn't know about Glaceon or Leafeon (and thus didn't recognize their sprites during evolution) or simply didn't notice what was happening, well... I hope you saved recently, as once an evolution is complete, there's no going back barring a reset.
It's even worse in Gen V, where a few players may not have even realized the The Dev Team Thinks of Everything, and added new Mossy and Icy Rocks to Unova; then, they unwittingly level up their Eevee in Pinwheel Forest or the basement level of Twist Mountain. On top of that, thanks to the new evolution scene (which removed the silhouette flicker in favor of a vortex effect), it's hard to tell just WHAT it is your Pokémon is evolving into until it's already too late.
As though we didn't have enough Eevee-related woes, in Gen VI, it got another evolution in the form of Sylveon. How do you get one of these? Eevee has to level up with level 2 Affection (from Pokémon-Amie) while knowing a Fairy-type move. This is never mentioned in the game, and it took a while after the game's release for somebody to figure it out. Actually getting your hands on Sylveon is frustrating enough if you didn't know this beforehand, but it's worse if you're trying to get an Espeon or Umbreon instead; if you happened to let your Eevee learn Charm or Baby-Doll eyes, and its Affection is high enough, the Fairy-type move-based evolution takes priority over the normal friendship evolutions. And, unlike Gen V, you have no way of telling what your Pokémon is evolving into until it's evolved.
As of Generation IV, some Pokémon only evolve if they know a specific move when they level up. Some of them make sense - Mime Jr. and Bonsly need to learn Mimic to evolve, and they are known for mimicking things. Yanma and Piloswine need to learn Ancient Power to evolve, and as their evolutions have prehistoric basis, that makes sense. Tangela into Tangrowth with Ancient Power, not so much. Evolving Lickitung into Lickilicky makes no sense whatsoever, as it needs to know Rollout to evolve, and that has absolutely nothing to do with any of the evolutionary line's abilities. But with Piloswine, it's worse than that; the only way it can learn Ancient Power to begin with is via the Move Relearner, so if you don't know about how to evolve it, you'll very likely never teach it that move to begin with!
One better to add to the induced confusion: Piloswine can learn Ancient Power in the earlier games, but it didn't evolve then even if it had the attack.
Magneton and Nosepass only evolve if leveled up in Mt. Coronet, which apparently exudes a magnetic field that affects the two magnet-based Pokémon. Tragically, nowhere in the game is it mentioned that Mt. Coronet is magnetic! An NPC in Platinum tells you that certain Pokémon level up at Mt. Coronet, but they never hint as to which Pokémon or that Mt. Coronet is magnetic.
There was exactly one hint in the original Japanese version, but it was removed during the localization process which renamed Tengam Mountain to Mt. Coronet for whatever reason.
In Platinum, you can catch Nosepass in Mt. Coronet, which could possibly be seen as a hint... though that's still a pretty darn vague hint.
The tradition continues in Generation V, where they evolve if leveled up in Chargestone Cave. Luckily, Chargestone Cave is obviously magnetic this time, but there is still nothing anywhere to suggest leveling your mon up there will do anything. Black/White 2 does add Nosepass to Chargestone Cave, at least implying an association.
Kalos' analogue to Coronet and Chargestone is Kalos Route 13. The only hints we really get are the nearby Power Plants and some lines by a worker of an electrical disturbance irritating the local mons.
It certainly doesn't help that the local mons were all Ground types (two of which have a high probability of having Arena Trap), have fun trying to train your Magneton and Nosepass there!
Baby Pokémon might count as this: you can only find some Pokémon by breeding their parents. This then requires a female evolution of the baby. Which Pokémon are only obtainable therein isn't really explained. To add insult to injury, in the case of any baby introduced from Generation III onwards this also requires the parent to be holding a certain type of incense. Using a Ditto will also work for breeding when you only have a male, but if you're trying to get certain egg moves (another aspect that isn't exactly mentioned in the games) you need the father to know the moves you want inherited, while the mother is the Pokémon you want it to be born as. Then there's also Volt Tackle, which can only be learned as an egg move if bred with a female Pikachu or Raichu carrying a Light Ball.
Breeding again, this time concerning on what pair of Pokemon can pair with each other. Specifically Egg Groups. A male and female Pokemon can only breed if they're of the same egg group, which is never mentioned anywhere outside of guides. This might make sense for Pokemon like Flygon and Noibat that cannot breed with other Dragon-types because they look and belong respectively into Insect and Flying Egg Group, but can be plain weirdin the case of Skitty and Wailord...just because they share the same Egg Group (Field). And what do you mean the humanoid Gardevoir is of the Amorphousnote usually reserved for blob-and-gas-like Pokemon group?
Certain Pokémon only evolve if traded while holding a certain item, or if leveled up holding a certain item. These items aren't labeled as influencing evolution, and many of them have a beneficial effect, so one might never realize they have a second use.
Others don't have any other effect, but are also extremely vague as to what they're used for. For example, the in-game description of the Protector reads, "A protective item of some sort. It is extremely stiff and heavy. It is loved by a certain Pokémon." This item cannot be found on a wild Pokémon, so there are no clues as to which Pokémon is being referred to. It's Rhydon, which will evolve into Rhyperior if traded while holding one. The Reaper Cloth has the same situation, although you might be able to get a minor hintnote After catching Giratina, if you return to Turnback Cave, an item will be waiting for you in Giratina's room on each return trip, which changes based on how quickly you get there. 16-30 rooms will give you a Stardust (an item that exists only to be sold), 4-15 rooms gets you a Rare Bone (the same, but more valuable), and the minimum 3 rooms will net you an extra Reaper Cloth, which further points toward it belonging to a Ghost type., while the Magmarizer and Electirizer, albeit with similarly cryptic in-game descriptions, do not, as they are sometimes held by the wild Pokémon that would use them. The problem wit those two is that some games, they are only found on the BABY form and at a 5% chance. So you might get a Magby with a Magmarizer but then you have to evolve it to Magmar THEN work through evolving it.
Tyrogue will evolve into one of three Pokémon based on whether its Attack and Defense are higher than each other or equal. Strangely, all three evolutions have similar stats and are all offensively-based. Wurmple evolves into one of two Pokémon based on a stat that's never alluded to in the game called personality values. While its evolutions can be caught in the wild, they won't know any offensive moves and will never learn any until their final evolution.
Karrablast and Shelmet. The two have some kind of relation in vaguely-worded Pokédex entries. Evolving them turns out to be more confusing. Not only do you have to trade them to evolve (which is a Guide Dang It in itself), but you specifically have to trade one for the other to evolve each. No other Pokémon evolve this way.
Then there's Mantyke, which has another unique evolution method... you have to evolve it by leveling it up with a Remoraid in your party. Although the evolutionary line is blatantly associated with Remoraid, it's still a stretch.
Evolution-related Guide Dang Its were finally fixed in Pokémon Black 2 and White 2, which allow you to call the professor on the Xtransceiver and ask her how to evolve any Pokémon in your party — but then unfixed for Gen VI which has no such specific hints.
In order to fully evolve Sliggoo into Goodra, you have to reach level 50. Sounds simple, except it has to be raining in the overworld for it to evolve. The only hints we get are that Route 14 (the first time you encounter Goomy) is raining often and that there is a sign that indicates one Pokemon evolves in the rain but never says which.
Inkay to Malamar needs to reach level 30 and requires the 3DS to be upside down upon level up. The only hint is Inkay's preference for reversing.
To evolve Pancham into Pangoro, it needs to reach level 32 while a Dark-type is in the party with them. While it isn't a big one, it's annoying to those who don't like overlapping types in their party and expected Pancham to naturally evolve without any additions.
Mega Evolutions have a good bit of this in certain aspects. One of the key items needed to achieve it is a "Mega Stone". Sounds simple, just get that Gyarados you have been using since learning it could go Mega, add stone and hit the Mega Evolve button. Now try to find the stone. Can't find it? That's because for only one hour a day, you are able to find the stones and some can only be found post-game after defeating Calem/Serena and then talking to Sycamore. Just not knowing those details alone is frustrating, but you have an hour at most to find whatever Mega Stones you were hoping to obtain.
It's the one hour of the day when Mega Stones can be obtained. Where can they be found? It turns out that the locations of most of them have nothing to do with the Pokémon they're used with. Are the Houndoomite and Manectite found in the route that Houndour or Elektrike can be encountered? Nope, they're on the other side of Kalos. Mawilite isn't found in Glittering Cave where Mawile is normally found; rather, it's hiding in a place you have no reason to return to. Tyranitarite and Aggronite are only found in a dead-end of a Gym (but at least that makes retroactive sense considering the Gym's type specialty).
Wonder why a crucial stat of your Pokémon's seems weak for no justifiable reason? That's because of poor Individual Values (essentially, genes), and/or bad Effort Values (essentially, stat-based experience gained when you battle different Pokémon). The existence of these stats is only vaguely alluded to in-game, and fully understanding the process and its various formulas by oneself without hacking and digging deep into the game files is likely an impossible task. Knowing these stats and understanding them is essential for breeding Pokémon for competitive battling. In fact, this is specifically an Internet Guide Dang It, as even Nintendo's official guides include only very vague information on IVs or EVs. It's worth noting that being aware of the EV and IV systems isn't required to beat the core game and even achieve 100% Completion, but considering that the game alludes to and references the hidden stat systems, it would appear that Game Freak expected some players to know about them at some point.
Bradygames and Prima Guides in Gen IV, particularly for Pokémon Battle Revolution, allude vaguely and indirectly to the EV training system, complete with hints on what to fight for what kind of EVs.
In Black 2 and White 2, you can call Bianca after defeating the champion to determine if any given Pokémon in your party has hit its EV limit. It's not much, but it's something.
There are NPCs in games dating back to at least Generation III that will give you relatively oblique indications of your Pokemon's IVs and EVs if you know how to interpret their replies. One such NPC will tell you whether your lead Pokemon has maxed out its EVs, saying that it has "put in a great effort" or something to that effect if it has (and giving it an "Effort Ribbon" pre-Generation V, which eliminates ribbons altogether). Another, usually found somewhere in the resident Battle Tower/Frontier/Subway, will tell you whether your lead Pokemon has any maxed-out IVs and what its total IVs are—not directly or even exactly, but by using a set of code phrases. Finding those NPCs and interpreting their dialogue is a Guide Dang It in and of itself, but it's better than nothing.
Ever since the Battle Frontiers were introduced in Emerald and Platinum, there has always been an expert breeder who can and will appraise your Pokémon. He does not tell the exact IVs, but will give you a vague average of your Pokémon's stat potential from "rather decent" to "outstanding", and will tell you which stat is highest, or which stats are tied for highest. In Generation VI, he'll also warn if it has 0 IVs in any stat.
This Pokémon has relatively superior potential overall. That's how I judge it, anyway. Incidentally, I would say the best potential lies in its HP. Although its Special Attack is equally good. Stats like those... can't be beat! But you won't get anywhere fast with this low of a Speed stat...
The Macho Brace introduced in Ruby and Sapphire doubles the rate at which your Pokémon gain Effort from battling. However, the games never exactly specify the mechanics behind it.
As of Platinum, the Battle Frontier has several Power Trainer held items that are like specialized Macho Braces. They give 4 EV in the stat they embody, and their description explicitly says which stat they help to raise. However, it is not mentioned exactly how much they improve your stat training/growth.
Power Lens: A Pokémon held item that promotes Special Attack gain on leveling, but reduces the Speed stat.
This has been somewhat 'resolved' in X and Y, as Super Training more or less explicitly shows your Effort Value spread and gain as you play it or fight Pokémon with the Training app loaded to your touchscreen.
The move Hidden Power is a huge pain because of IVs. Why, you ask? Because the move's type and damage are both determined by the user's IVs. Granted, Hidden Power isn't that great of a move overall, with a maximum base power of 70 if your Pokémon was somehow blessed with the best possible IVs... but when you consider that nearly every Pokémon can learn it and the move's type can be anything except Normal and Fairy, you kind of understand why it's used a lot as type coverage in competitive battling. On the other hand, ever since Ruby and Sapphire there has always been a Hidden Power specialist in every region who will appraise your Pokémon and tell you the type of your Pokémon's Hidden Power. However he still doesn't tell exactly how strong that Hidden Power is.
The "how strong" part was changed in X and Y, now that Hidden Power's BP was fixed at 60. The type of Hidden Power is still determined by IVs, though.
On that note... So you just booted up a new game of Pokémon Sapphire Version, and you're experiencing it for the first time. You check out the new "SUMMARY" screens, which are incredibly detailed when compared to the "STATS" screens of the previous games. And what's this? "JOLLY Nature"? If you're instead playing a game of a later generation, this will be accompanied by something like "Loves to eat". Well, isn't that adorable? Flavor Text is so cute. ...Except this isn't Flavor Text. These are what are called Natures (introduced in Generation III) and Characteristics (introduced in Gen IV). Natures will raise a particular stat by 10% in exchange for lowering another one (with the exception of five natures known as 'neutral' natures). Characteristics, on the other hand, don't change or affect anything and are simply informative. They let you know what a Pokémon's highest stat is (measured by IVs, themselves a Guide Dang It). You'll absolutely need to know them in order to have any kind of success in high-skilled battle institutions or competitive play.
Here's where the nature (heh) of the Guide Dang It gets really ridiculous: Not only are you never told what any of this means, Natures aren't even self-defining or intuitive on their own. Sometimes they can be reasoned out. The "Timid" nature raises Speed but lowers Attack. Others are impossible to guess (Modest? Raises Sp. Atk and lowers Attack, obviously). Though once you've figured out one, you can usually figure out its opposite. The opposite of Timid is Brave, and sure enough the Brave Nature lowers Speed and raises Attack.
Characteristics, on the other hand, are even worse. If a Pokémon's Speed stat is as high as it can be, what would you expect it to say? "Likes to run"? Close! That's second best. The absolute best Speed stat is accompanied by the text... Alert to sounds. Unfortunately, you could also get the latter text with a speed stat of 1... if the other stats are all zeros!note The texts are based on the highest stat's remainder when divided by 5. Not 4 or 8, 5''. Unless you look up a whole chart, don't expect to figure this all out any time soon.
To their credit, from HeartGold and SoulSilver onward, Game Freak added a (very) slight tint to a Pokémon's stats screen showing which stats were being raised (in red) and which were being lowered (in blue). (A Pokémon with a neutral nature will not have this, of course.) Of course, noticing them and figuring out what they meant (you might think red was bad and blue was good) still has shades of this trope, but it seems this was added in for players who already knew what Natures were and simply saved them the trouble of memorizing which one did which.
The entire move list in the first generation of games was a Guide Dang It, because nowhere in the game did any of those moves get their effects described. So you really had no idea what that new move Charmander just learned actually does, you just knew it's a Fire attack with 15 PP. Is it stronger or weaker than that other Fire attack it just forgot? Who knows?
And still, in Generation II, you don't know if a move is good or not until you actually learn it. Obviously, when you replace a decent move for Swift (60 power, Normal type and ignores accuracy and evasion modifications), this can be frustrating when you deleted a 100% accurate Normal move with 85 or so power.
The ins and outs of breeding for movesets is a Guide Dang It all on its own. Breeding can result in babies that know moves from their parents, which results in expanded move pools for most Pokémon. However, which moves can be inherited aren't told anywhere in the game. Some of them can be inferred (it's easy to figure out that you can breed Thunderbolt onto any Pokémon who can learn it via TM), some of them aren't that surprising (Mud Shot can be learned by an awful lot - but throwing mud around isn't the most mindblowing technique), but some are positively mind-boggling (Aron, a Rock/Steel type that weighs over a hundred pounds and eventually evolves into something weighing nearly 800 pounds, can inherit a move called Aerial Ace, a swordsman-inspired move). On top of that, there's the rule that many guides overlook, in that a baby will inherit a level-up move if both parents know it. Valuable for Pokémon with wildly divergent movelists upon level-up (like Seedot) or anyone looking to breed for Tournament Play (which sometimes imposes level restrictions, preventing you from acquiring moves via Level Grinding).
There's the Generation IV National Dex. It is unlocked by seeing (not catching) every Pokémon in the Sinnoh Dex (150 in Diamond and Pearl—seeing Manaphy, the 151st in the Sinnoh Dex, wasn't necessary—and 210 in Platinum), and to help make this easier, they made it so that every non-legendary Pokémon in the Sinnoh Dex could be seen in a trainer battle at some point during the game. Furthermore, to increase the likelihood that you'd have to beat the game (or come up just short) to do so, the only trainer with a Garchomp, Spiritomb, or Milotic (and in Platinum, you can add Togekiss to this list) is the Pokémon League Champion. However, they didn't always make it entirely evident where to find some of these. Examples of Pokémon that could easily be missed:
Wormadam—in the party of a trainer on a part of Route 214 that you might not even end up visiting at all (in between Veilstone City and Lake Valor, since the first time you visit Lake Valor for plot purposes, you'll be coming from Pastoria City), whose gaze can easily be avoided as she isn't facing the main path.
Riolu—in the party of a trainer at Ravaged Path. Platinum remedied this by giving one to another trainer on Route 217, but given the wide-open nature of that route, it's still no sure thing. And the only way to get one for yourself is to go through a sidequest in an optional area while keeping a slot in your party open (which also means no catching Pokémon while you're there) and then hatching an egg.
Drifloon, which in Diamond and Pearl only appeared in a skippable trainer battle inside Fantina's gym, could be annoying to find as well if you didn't know that one and only one appears in front of the Valley Windworks every Friday. One appears in Amity Square with a Trainer and you can talk to it, but that doesn't count as viewing it...
A similar thing happens in Generation V with Larvesta, which can only be obtained by either breeding a Volcarona (single one per game, in an area you'll have to visit at least twice for plot, so not as bad) or getting an egg from a trainer at the far end of an otherwise optional, dead-end route. No trainers use it in battle, and it's part of the Regional dex. Fortunately, the National Dex is opened by beating the Champion instead.
In sequels at least, Volcarona is treated like stationary legends - if not caught, it respawns upon Hall of Fame.
Palkia (Diamond)/Dialga (Pearl)/Manaphy (Platinum): In the original two, the Pokédex data for the legendary not appearing in your game could be obtained by returning to Celestic Town after Spear Pillar and talking to Cynthia's grandmother. In Platinum, both Dialga and Palkia are seen (and uncatchable) on your initial trip to Spear Pillar, but the expansion of the Sinnoh Dex to 210 means that data for the hidden number 151, Manaphy, must be collected. You can find this data in a similar fashion, but a different place: a book in Mr. Backlot's mansion.
Rotom. Without the guide letting you know you can only catch it in the Old Chateau at night, and that you need a certain key to unlock its various forms...you'd pretty much have no idea it even exists.
If you examine the TV during any other time, it at least hints that there's something special about it... However, it is kind of vague, and if you didn't already know about Rotom then you'd have little reason to examine the TV in the first place.
In Generation VI, Rotom can be found in garbage bins at the Lost Hotel, but only on Tuesdays. Rotom cannot be "seen" in the Kalos Pokédex without catching one (meaning you can't consult the in-game Pokédex as to where they are), and there is no guarantee that any of the garbage bins would contain Rotom. While some of the garbage bins wiggle, hinting that you should check them, the ones that stay still may have Rotom in them too. On all other days of the week, they contain Garbodor.
Before all of this, there was the originalGuide Dang It of the series: physical versus special moves. You can hammer your way through the game without knowing the difference, but if you want to fight well, it's the very first thing you have to understand. But nowhere in the first three generations of Pokémon games are the nine physical and eight special types actually listed — not even at the trainer schools! It does get listed in Earl's Academy in Pokémon Stadium 2, but it's fair to assume not that many people delved too deeply into what was a pretty minor part of the game. Generation IV uncoupled physical/special from move types and added icons to each move showing which of the two it was, so it's no longer an issue.
The entire Pokémon series is pretty much one giant Guide Dang It, but one huge one is the whole Elemental Rock-Paper-Scissors. The game does explain about super-effective and not very effective hits, and it gives some tips before Gyms, but you don't see a chart anywhere detailing the full list of what resists and what's weak to what. Some can be intuited, like Fire against Grass or Bug, Water against Fire, or Electric against Flying or Water, or Ground against Electric, but quite a lot of them make you scratch your head. Dragon weak to Ice? Rock weak to Ground? Poison weak to Psychic? Dark weak to Bug? Fighting weak to Fairy? You can only find this out through trial and error, which can take a while. Luckily, you can get a type-matcher app in the Generation IV games which helps, and the Generation I remakes had a type-matcher guide if you left the L-Button=Help option on - and the type guide is only available in battle, anyway which adds a new layer of Guide Dang It.
Well, nowhere except the original Generation I instruction book, anyway—although it wasn't very clear that "Bad" was supposed to mean "no effect" in that chart. And of course, it didn't cover Steel and Dark (or Fairy), or any type-matchups that were changed for Generation II.
What's more, the type matchups in Generation I were glitched so that while according to the manual, Ghost was supposed to be super-effective against Psychic, it was in fact completely ineffective. Which rendered the already powerful Psychic type a Game Breaker, since it had no weaknesses outside of Bug, a type with few useful Pokémon or moves at the time. Good luck figuring out why your manual was lying to you.
Transferring your Generation II remake Celebi and/or Beast Trio to get your Zorua/Zoroark. And even if you know to do that, you need to use the Relocator if you want to transfer them before getting the National Dex, and finding it within the maze of buildings that is Castelia City isn't the easiest task.
Purifying Shadow Lugia in Pokémon XD: Gale of Darkness. It's said multiple times throughout the games that it cannot be purified, and you're led to believe they're right; using it in battle, giving it Scents, even putting it in the Purify Chamber does absolutely nothing to drop its Heart Gauge. Well, to purify it, you need all 9 sets of the Purify Chamber to be at full tempo, and then you need to put Lugia into any of the sets. This instantly drops the gauge to zero, giving you the ability to purify Lugia. Now, if just putting Lugia into the Chamber regularly doesn't do anything, how the hell would you know to have 9 perfect sets going at the same time?
What makes this worse is the fact that the Purification Chamber itself is otherwise fairly intuitive. Having sets at max tempo does help you purify more quickly, so if you've been using the Chamber much it's likely that you already have at least a few sets at max tempo. Still, nothing at all in the game hints that you need all sets at max tempo to purify Lugia.
And then there's the coup de grâce - Lugia's heart gauge will revert back to full darkness if removed from the purify chamber, meaning that you can't finish the process like every other Pokémon. The trick? You need to visit the physical in-game location of the purify chamber.
The sidequest to trade with the other Gen. III games in Fire Red and Leaf Green. The first part is pretty straightforward; beat the Grunts, get the Ruby, and catch Moltres early if you want. The second part is nearly impossible to figure out (unless you can read Braille). You're told that the Team Rocket base requires two passwords, and you already have one. You are then expected to go two islands ahead and run all over looking for an ancient ruin, then Cut the door open and fall through holes in the right order. You are given no direction (in your language, at least) at all, then once you find the Sapphire you are robbed and THEN storm the Team Rocket base.
One very, very small one, and only for those of us for whom Pokémon was one of our first-ever Role-Playing Games. The games start you off in your room in the upstairs part of your house. Going downstairs is intuitive, but nowhere does it say that the little floor mat marks the front door of your house. It's even somewhat Lampshaded in the help function for the Generation I remakes — one of the topics you can look up reads something like: "I can't get out of the house! Help!"
Similar to that last one is the Olivine Lighthouse in Pokémon Gold and Silver and the remakes. There's one section that involves a blind leap out of one of the lighthouse windows, and without looking it up, it can be frustrating trying to find your way off that floor.
A more mild example. While one can muddle through by trial and error, Berry growth in Generation IV has driven some tropers to create cheat sheets of Berry growth. As silly as a full-blown spreadsheet sounds, the developers got rather enthusiastic with their Berry system. Each Berry plant has a total growth time between 8 and 96 hours and a moisture rate (between 4 and 35) that is deducted from the soil moisture count each hour. This count is set to 100 at planting and reset to 100 whenever the plant is watered; the final yield of the plant decreases by one berry for every hour the plant spends at zero moisture. Thus, each plant has to be watered with a certain easily calculated frequency to ensure maximum yield. To keep things from being too simple, the dev team added Mulches to either reduce the water requirement while increasing growth time, or decrease growth time in exchange for more frequent watering. With this twist, Berry plants now have three different growth times, and three different minimum watering frequencies; the Guide Dang It kicks in when you realize that the moisture consumption rates aren't even listed on the more popular Pokémon wikis, and are instead found exclusively in obscure internet forums.
Possibly Ditto's role as the universal breeder. The only hint is that the only move it knows lets it transform into other Pokémon.
Also, nowhere in the game hints that non-legendary Genderless Pokémon, which normally can't breed with anything, can produce eggs when bred with Ditto. Or that the one thing that Ditto can't breed with is its own species.
On that matter, egg groups, which indicate what Pokémon species can breed with one another. Like elemental types, a Pokémon can have up to two egg groups, but they're quite independent from types: two Pokémon can share a type, yet belong to different egg groups; conversely, two Pokémon can be in the same egg group(s) while having no type in common.
It can usually be inferred that Pokémon of similar body shape can breed with each other (for example, its easy to see that Houndoom and Manectric can breed with each other since they are both based on dogs). However, the game does throw a few curve balls here and there. For example: Gardevoir looks like a woman in a dress so it should be able to breed with other Pokémon that look like people (the Human-like egg group). Right? Wrong. Its actually in the Amorphous egg group. Meaning that it breeds with things such as greasy piles of sludge, ghosts and flatfish. Archeops is said to be the ancestor of other bird Pokémon, and indeed it can breed with them... But it can also breed with things such as crabs, barnacles and jellyfish. Aggron is an 800 lb. steel-plated Triceratops, so it would be logical that it could be bred with similar monstrous beasts... and also sheep, living icebergs, and frilled lizards. Eelektross is a giant ocean-dwelling lamprey, which would seemingly indicate it belongs to the three "Water" Egg Groups. Nope, it's none of them at all, instead being in the Amorphous Egg Group along with Gardevoir. The system gets very strange at times.
In HeartGold and SoulSilver, there are the Shiny Leaves. You need to collect five leaves on a single mon and have Ethan/Lyra make a crown out of them in order to add a trainer star to your card. Seems simple enough, but the routes your 'mon can obtain said leaves is dependent on their nature, so expect to comb both Kanto and Johto's grassy regions and interact with your 'mon to see if a reaction occurs. To make it worse, nowhere in the game hints to these leaves until you actually find out, at which point Ethan/Lyra give a little info and then leave the rest to you.
HGSS also have the Gym Leader rematches. You can rematch any Gym Leader in the game, with a souped-up team...if you have their phone number. Talking to them in their Gyms won't get you the number. For most of them, you have to find them outside their Gyms, and then they'll give them to you. Most of them have either a tiny window of time in which they're available (Jasmine, Lt. Surge), while others appear well off the beaten path or in places you aren't likely to revisit (Morty, Bugsy). But the absolute worst is Blue. You can scour the world and you'll never find him. To get Blue's number, you have to have his sister massage one of your Pokémon seven times, while said Pokémon is at maximum happiness, then she will give you Blue's number.
Black & White 2: The Dropped Item sidequest. You may even miss picking up the key item on your way through the game, not even knowing it was there. The item in question is a Xtransceiver that belongs to a person named Yancy (if you chose the male Trainer) or Curtis (if you chose the female Trainer). You have to scour the main hexagon of Unova, for 15 specific areas, and on a specific tile to trigger a call from them. After that (getting called ten times by Yancy/Curtis), comes the second part of the quest. You have to call them in the above mentioned 15 areas (not on the same tile, just being in the route/town counts this time). Where you can call them is randomized, meaning you have to run/fly around Unova to the designated areas, and their number will disappear after every call (and you need to either wait a day or go to a different area)...and it takes a total of 50 phone calls to get your ultimate reward: unlimited daily trades in which Yancy/Curtis will trade a Lv. 50 'mon (with its Hidden/Dream World ability) for any 'mon. Good luck trying to figure all of this out without the aid of an online guide.
There are 5 Berries which restore 1/8 of HP, but confuse Pokémon who hate its taste. This is not as much as problem in Generation III and IV, where the tag options exist (bag description of HGSS outright states the taste), but Generation V? They say "If held by a Pokémon, it restores the user’s HP in a pinch, but will cause confusion if it hates the taste.", but nowhere in the game it is stated which berry has which taste or even what taste is, so unless you've played older games or feel like experimenting... Of course, they're inferior to Sitrus Berries, which restore 1/4 of max HP but with no chance of confusion and they take effect if the Pokémon's HP reaches below half, not in a pinch. But the game never tells you that either.
A number of Abilities that Pokémon can have also have effects outside of battle... which are never alluded to anywhere:
Compoundeyes: If a Pokémon with this Ability is in the lead spot, wild Pokémon you meet that have a chance of carrying an item get an increased chance to be carrying that item.
Cute Charm: If a Pokémon with this Ability is in the lead spot, wild Pokémon you meet have an increased chance of being the opposite gender of that lead Pokémon.
Intimidate and Keen Eye: Lowers the chance of encountering lower-leveled Pokémon.
Static: If a Pokemon with this Ability is in the lead spot, any Electric-type Pokémon that have a chance of appearing will show up more often.
Similarly, but less obtusely, Magnet Pull's ability to attract Steel-types is never actually explained, but it's pretty obvious when you think about it. It can also cause Magnemite problems in Ruby, Sapphire, and Emerald if you enter New Mauville with a Nosepass, or give you lots of Klink and Ferroseed encounters if you enter Chargestone Cave with a Magnemite.
Synchronize: If a Pokémon with this Ability is in the lead spot, wild Pokémon you meet have an increased chance of having the same Nature as that lead Pokemon.
Lightningrod: In Emerald, having a Lightningrod Pokémon in the party lead increases the rate that NPC trainers will call you.
Flame Body and Magma Armor help eggs to hatch faster.
Join Avenue in B2W2: Recommending shops to people walking down the avenue can be tricky. Some requests can be as vague as "I want to go there!" or "Where do you think I want to go?" The player would have to look up exactly where in order to avoid them leaving out of disappointment. The only one that's safe is "Please take me to a shop you recommend" where any shop is allowed, which is not the case for the similarly phrased "Take me somewhere! Anywhere!".
The Boutique located in Lumiose City won't allow you in until you are stylish enough. They don't mean that you have to look pretty, rather, you need to get a hidden stat to a certain level before they'll let you in. There are a number of ways to increase this stat around Lumiose, but nothing actually tells you that these things make you more stylish and the dialogue when you get kicked out of the boutique seems to suggest that you basically just need to visit every building in town.
On a similar note, a number of other services are unlocked or have their prices reduced as your style rating goes up. That old guy in the stone shop selling Mega Stones for 500,000 Pokédollars? Get your style rating up high enough and it drops to 10,000. The Art Mueseum? Gives you the Audio Guide to the gallery's pieces for free rather than require a token small payment of 200 Pokédollars. The Hairdresser? For girls, she'll eventually treat you as a regular customer (and get ...distracted... by how enchanting you look), which unlocks a few more options of hair customisation.
The final O-Power, the highly useful Hatching Power, is difficult to find, let alone get. To get it, you need to max out your style and go to Café Introversion, where Mr. Bonding will be if you did everything right. Knowing that you have to max out style, and even where to find the last O-Power, is not hinted at anywhere.
What about Trainers and their Pokémon's movepools, during Trainer Battles? In Gen I and Gen II, the majority of their movepools are level-up moves, which means that at the very least, you've got a pretty good idea what type their Pokémon's moves are most likely to be. Only Gym Leaders would use TM moves. Fast forward to Gen VI, and now it seems like they'll source their Pokémon's moves from anything, including TMs and Egg Moves. (Sometimes even from cheating.) And there's absolutely no telling what abilities their Pokémon will have too without consulting Bulbapedia, creating another avenue to screw up your strategies. And this is not just for Gym Leaders, but regular trainers too.
However, Serebii's Pokéarth page has a list of trainers in every area between the DS era games that also shows what moves their Pokémon have. It also lists the abilities for Generation V trainers but not those of Generation IV. For Generation VI, it lists the Pokémon but at the moment not their moves.
Trying to figure out which Pokemon learn which moves can be frustrating because sometimes there seem to be no rules as to what moves a Pokemon can learn at all. Moves learned by levelling up are fairly predictable and tend to be within the Pokemon's own type. As for T Ms, Pokemon understandably can learn almost any TM move that is the same elemental type as they are. TM moves outside of a Pokemon's own typing is where it really gets confusing. They can be anywhere from reasonable (Weavile can learn Shadow Claw) to a bit of a stretch (Charizard can also learn Shadow Claw for some reason) to unreasonable (Geodude can learn Flamethower??) and sometimes just impossible (Alakazam, who is tailless, can learn Iron Tail.). You can't know what moves someone might have without looking it up.
Invisible items have been scattered around the overworld since Gen I. Some of them are on conspicuously odd bits of terrain, but others are in completely random places, and there's no way to know anything is there unless you stand facing that exact square and press A, or use the Itemfinder.
Speaking of the Itemfinder, in its original incarnation in Generations I and II, it only indicated the presence of a hidden item, but did not drop hints as to where it was. Happy easter egg hunt! Thankfully later incarnations of the Item Finder/Dowser make it more user-friendly by letting it indicate which direction the hidden item was. Even so, there is still yet another undocumented Item Finder behaviour in Fire Red and Leaf Green - At certain special locations (Where Mr Fuji, the two Snorlax, Giovanni (in Viridian Gym) used to stand before leaving their location, as well as tiny patch of land by the Cape Brink's pond and Navel Rock), standing exactly where these folks used to be and using the Itemfinder will produce a unique reaction that reveals a special item (Mr Fuji's spot in Lavender Tower has a Soothe Bell, there are Leftovers where the two Snorlax used to be, Giovanni was standing over a Macho Brace the whole time, that tiny patch of land by the pond has PP Max, and if you had access to it, you could find Sacred Ash on Navel Rock, in addition to the one Ho-Oh holds in Emerald.).
Before FRLG, Stick and Thick Bone had description of vendor items, while in reality they increases Farfetch'd's critical hit ratio and Cubone and Marowak's Attack, respectively. They were one of the few items to get description changed in Emerald, where it was corrected.