With hundreds of Pokémon as well as having a metric crapton of characters, don't be surprised that there will always be a character that fans want to check on their cartridges and release them from their game.
Anime & Manga
- May's Squirtle made a bad first impression with the entire cheapness surrounding its first contest victory and the hate hasn't stopped since. The show already had a beloved Squirtle, making May's a Replacement Scrappy that was possibly particularly disliked by people who prefer Ash's original team. When May returned in Sinnoh, her Squirtle was revealed to have evolved into Wartortle, its brief appearance showing it to have potentially been Rescued from the Scrappy Heap due to no longer being the crybaby it started as.
- Tobias. Even among Ash's League Rivals, he really stands out in how hated he is. Firstly, he has extremely little buildup before his battle with Ash, only appearing in a few cameos. Even fellow Diabolus ex Machina trainer Cameron, who is also widely hated, at least had several episodes with him as the focus before his victory over Ash. Then there's how he defeats Ash. He wins 6-2, and the only two Pokémon he uses are Latios and Darkrai, a Legendary and Mythical Pokémon respectively. It's never revealed how he obtained them, he serves no purpose other than to eliminate Ash, and completely vanishes after he wins the league, with Cynthia remaining Sinnoh Champion in later appearances with no mention of him. Many fans took the entire thing as a huge slap in the face, and Tobias' victory solidified fandom opinion that Ash Ketchum would never win a Pokemon League so long as the writers had anything to say about it, a trend that wouldn't be bucked for another decade.
- To put it another way, fans are naturally inclined to hate any character the writers use to stop Ash from winning a League, but you'll find most of the League Rivals have defenders who will argue that Ash bears responsibility for his defeat given the choices he made and/or his own flaws as a trainer.note You'll be hard-pressed to find anyone making those arguments in the case of Tobias. It's possibly the one loss fans will let Ash off the hook for, or at least the one they're least likely to bring up as proof he's a bad trainer, since he still gets closer to beating him than anyone else in the Sinnoh League.
- Ash's Pignite gets a lot of flak for recycling Charizard and Infernape's story arcs in a less interesting way.
- The Mewtwo from Pokémon: Genesect and the Legend Awakened. She is quite unpopular with the fandom for filling a role that many believe could have been filled by the well-loved established Mewtwo character from Pokémon: The First Movie and its sequel, for having a voice and personality that contrast sharply with said original Mewtwo, for having no explanation as to how a second Mewtwo is even possible, and for generally being the Fleeting Demographic Rule personified. Even Miriam Pultro, her English voice actress and One of Us, knew that the role would be controversial. When Mewtwo returned for Super Smash Bros., they went with the original portrayal instead of this particular one.
- Scott from Emerald, due to his tendencies to appear or contact you at odd and inconvenient times.
- If you are in the Master Rank of the Beauty Contest, and if a woman named Tanya and her Chimchar named Chimpy are in said Contest, it is very likely that she will win because Chimpy's Beauty is ridiculously high compared to your Pokémon, no matter how many Poffins you feed it.
- Mindy of Snowpoint City is hated for offering the player a Haunter for a Medicham while slapping an Everstone on said Haunter, making it incapable of evolving into a Gengar, a powerful, fan-favorite trade evolution, from said trade. Even her dialogue after the trade suggests that she's trolling you.
- The Spiky-Eared Pichu and the Pikachu-Colored Pichu in Pokémon HeartGold and SoulSilver. Not only does the former replace Celebi in an event, but there's pretty much nothing special about them except for their appearance and odd moveset. The Spiky-Eared Pichu can't evolve and can't be transferred to the fifth generation, meaning there's absolutely no reason to even bother with it. (The Pikachu-colored Pichu is at least a "shiny" Pokémon, which are normally rare in the games, though it is still considered a waste of an event as they are already technically accessible in-game, unlike most event Mons.)
- There's a girl riding a Stoutland throughout Heahea City who prevents you from visiting the Hano Grand Resort until Olivia is beaten because she wants to find every last hidden object in the city and the bridge connecting it to the resort, with her selfishness and role as a roadblock both contributing to her hate. By the time the resort becomes available, she'll be on the beach planning to look for even more items, not even apologizing for her hindrance earlier on.
- A Wobbuffet is a particularly devastating addition to a player's team and can prove particularly effective in Max Raid Battles... if the trainer is competent. Cabbie Alfie, however, is anything but. His Wobbuffet knows Safeguard, Amnesia, Counter and Mirror Coat. This wouldn't be so bad, except [A] Max Raid Pokemon can dispel stat changes, effectively breaking Amnesia, [B] the moves of Max Raid Pokemon are randomly decided, making Safeguard less viable and the reprisal moves a gamble, [C] the Pokemon's lack of Focus Band and HP keep it from taking a hit and [D] the Wobbuffet has no idea which move to use against the Max Raid Pokemon. When a Magikarp is infinitely preferable to you and your Pokemon, you have utterly failed as a trainer.
- For a similar reason, Gentleman Martin and his Solrock are widely disliked as Raid partners. Instead of attacking the Raid Pokémon, it prefers to boost its defences with Cosmic Power... except, because Raid Pokémon can remove stat boosts when they feel like it, Solrock will have essentially wasted its turn. Even when it does decide to attack, it uses fairly weak attacks and isn't packing a Focus Sash, so it'll rarely contribute to breaking barriers or actually doing damage. Martin is so widely hated, even Pokemon.com's official Max Raid Battle guide made fun of him and his "worthless Solrock".
- Sordward and Shielbert are heavily disliked for their hideous designs, Upper-Class Twit personalities, and being extremely disappointing and anti-climactic villains.
Gale of Darkness
- Michael's little sister Jovi. You're tasked with finding her at the beginning, she follows you around until you've collected the Machine Part, and insists you're her assistant the whole time.
- Cresselia in Pokémon Mystery Dungeon: Explorers. Not only do you have to escort her to the end of the dungeon (a frustrating task already with regular Pokémon, and she's a Legendary), but she also has the habit of wandering over lava (Which burns her and ends up wasting your Reviver/Heal Seeds). She's also involved in the final battle against Darkrai, in which she's incapable of doing much of anything to him other than fighting off his Mooks.
- From the same PMD games, Chatot. He's a Know-Nothing Know-It-All, and the infamous Perfect Apples incident proved him to be a Horrible Judge of Character. To summarize Much of the rest of the team don't like his attitude much either, and even Wigglytuff won't hesitate to call Chatot boring. That being said, he's still a loyal member of the Guild and willing to take a hit for his team, and Explorers of Sky shows some of his Hidden Depths in one of the bonus episodes, but good luck finding anyone who's willing to defend Chatot's idiocy in this instance.
- Pidgey, Rattata, and Weedle: annoyingly Com Mon Pokémon who you have probably caught a crazy amount of. Good for power leveling with a Lucky Egg. Terrible for anything else. And with Mega Evolutions six generations down the line, their evolved forms aren't much better. Making matters worse is that they have higher flee rates (20%) compared to rarer Pokémon, making them likely to cause further frustration. Thankfully these became much rarer when more common encounters debuted in future generations, though weather-boosts may result in these once omnipresent creatures running amok through the area (especially in Partialy Cloudy weather). At the very least, they do have ''one'' use - since they evolve with far less candies than other Pokémon, they can prove useful for mass evolve EXP grinding, preferably in tandem with a Lucky Egg.
- Zubats also count, especially since they're hard to capture and evolve into the somewhat mediocre Golbat. The Gen 2 update alleviated this somewhat with the addition of Crobat, which is respectably strong, and allowed Zubats to evolve with just 25, making them more reliable for power leveling.
- As of Generation II, Murkrow. Depending on where you are, Murkrow can spawn everywhere and can be considered a devil in terms of capture rate. Some players without good resources can waste more than 10 balls to catch one. Murkrow itself is rather substandard compared to even fully evolved Com Mons, and even though it can now evolve to Honchkrow, doing so is a costly endeavor requiring a Sinnoh Stone, compared to the cheaper evolution requirements of other Com Mons that can be spammed for quick XP. The release of Team GO Rocket Invasion, however, also involves beating a 8k+ CP Snorlax with Zen Headbutt that decimates Fighting types on your own. Honchkrow, being one of the more common Dark types (which double resists Psychic) but stronger than the likes of Alohan Persian or Raticate, becomes a great counter.
- Aipom also counts, as not only does it have an abysmal catch rate, the only trophy it counts towards is much easier to get by catching Rattata, Pidgey, and Sentret. Not to mention, its evolution doesn't fare much better in battles.
- Gen II added Sudowoodo, a rather mediocre Rock-type with low CP which, unlike other Pokémon like Murkrow, has no evolution in future Gens to look forward to. The problem isn't so much running into them in the wild, since they are rather rare, but the fact they hatch from 10km eggs. It can be pretty disheartening to put in the work hatching that egg hoping for a Dratini or a Chansey, and get an underwhelming Sudowoodo instead. There's also its pre-evolution Bonsly, but it requires you to walk 15 kilometers total first to evolve it.
- The trading update added 7km Eggs which contain Alolan Pokémon. Among the possible outcomes, Alolan Meowth and Sandshrew are distressingly common, and quickly earned the ire of players gunning for the rarer Pokémon.
- Later on, babies are migrated to 7km eggs as well, so there's a fat chance of hatching Igglypuff and Pichu from 7km eggs, which are even worse despite the odds of being able to hatch rarer babies.
- Flower Crown Eevee was added to the Research Breakthrough as the only Pokémon in the roster during the two months leading up to Halloween 2019, leading to intense backlash on social media (and possibly becoming the most widely-panned Research Breakthrough reward to date). The intent behind the two-months-straight of Eevees, according to Word of God, was to allow players the chance to acquire Flower Crown variations of each Eeveelution- which is hard when there are exactly eight (including the as-of-yet-unreleased Sylveon) and three of those evolutions are tied to RNG without abusing the nickname trick.
- Flower Crown Eevee's distaste in the fandom was almost matched by the introduction of Woobat as the Research Breakthrough reward for February 2020. Being a new Pokémon and giving 13 candy per catch without Pinap usage was, needless to say, not enough to smooth over the month's breakthroughs being solely devoted to a Pokémon with no PVE relevance, barely any PVP relevance, and taking up space from other, more needed Pokémon - especially as the reward for the first full month to have the Go Battle League ranked PVP system available worldwide, with countless meta-relevant Legendaries existing that could have filled the slot instead.
- Hat-wearing Pokémon in general have gone on to become widely loathed by the playerbase. While they were tolerated early on as any of these kinds of Pokémon could be evolved (allowing hat-wearing Raichu or Vaporeon or a Blastoise wearing aviators), the novelty began to wear off heavily with the infamous Flower Crown Eevee mentioned above and any goodwill the playerbase still had toward them was evaporated when nearly all future hat-mons had their ability to evolve removed, effectively making the hat-mons useless storage clutter. The Party Hat Starters of the 2020 Pokémon Day event were especially hated for not only being unevolvable but also becoming the only Pokémon that could be hatched out of 7k eggs for the duration of the event, unexpectedly interrupting an event where fossil Pokémon were hatching from them instead.
- Pikachu. Niantic probably thought it would be a cute Mythology Gag to make Pikachu extra-resistant to pokeballs, leading to trainers exhausting their pokeball supply just to catch one. They were wrong.