Many of the rare monsters in Monster Rancher have long, complicated methods of unlocking. Some of them can be stumbled into by accident, but some of them are Guide Dang It to the extreme:
Unlocking the Serket (a special sub-species of Arrowhead) in Monster Rancher Advance 2 requires you to create a "purebred" Arrowhead with a fully Arrowhead family tree, in which each Arrowhead was A-level or higher, and combine it with a Joker at B-level or higher. And there's couple of these in the same game.
Advance 2 has several example that is taken from Continuity Nod to previous games, and the anime. Particularly the Ragnaroks, which comes from the Monol - Gali myth that origin can be traced back to the first game and Ragnaroks card entry in the second game. Then there's Phoenix, which requires you to get 5 orbs from Mocchi, Golem, Suezo, Hare, and Tiger, referencing the Phoenix in the anime whose soul was split into those five monsters.
Unlocking the Doodle species in the same game required you to raise a Garu-type monster to a specific level, with certain stats, and have it look in a very specific place in one of the ruins, after having unlocked all the other monster species.
Getting both the Niton and the Undine species in Monster Rancher 2 required you to raise a monster of the Hopper species to Rank B, then have it on your farm during December—it had a random chance of digging up hot springs. Of course, just about the only way to figure this out is via pure chance. To make things even worse, while getting the hot springs automatically nets you an item that you can use to fuse and make an Undine, nowhere does it say that you've now unlocked Niton—the only way to find that out is by trying to regenerate a disk with a Niton on it.
Perhaps the "best" example: Unlocking the Beaklon/Beaclon in Monster Rancher 2 requires you to raise a monster of the Worm species to be four years old (by itself, this is already a Guide Dang It). Over the course of its life, it must be raised in a perfectly "even" fashion (never "strict" or "pampered"), it must never get stressed, and it must consume 30 Cup Jelly items over the course of its life. On the fourth week in June, if all of the above conditions are met, it has a chance of metamorphosing into a Beaklon. If you screwed up somewhere—and it's hard to tell—it only turns into a Worm hybrid monster or maybe you did it right and are unlucky. Plus, your Worm must not reach a rank higher than C, or else you've wasted a meticulous opportunity.
On the bright side of this, you are NOT required to send your monster in a tournament or errantry for this to happen. Therefore, while you'll be spending 4 long years in-game time raising your Worm at a meticulous manner in order to have it hopefully hatch into a Beaclon, at least your Worm won't have to suffer hardship to prove it.
There is a sort of silver lining in the Beaklon in that trying to get one you might stumble into another Guide Dang It monster, Ducken. For that monster, you need to get the 4 doll parts from Randomly Drops of Cup Jelly, and do it again after finding the glue from exploration in one of the most thankfully suspicious places of the exploration area.
Unlocking Joker, which needs you to explore a well-hid area and wait for a small chance to get the item. Can lead to repeated Soft Reset if it won't spawn.
Unlocking Ghost, which involves letting your monster, any monster die, in which, after five years, you get the chance to upgrade its if you have enough money (which would be a lot). Then, after another five years, you get another chance to upgrade the shrine, and you'll need a whole lot more money for it. Say yes, and you'll find a stick, which you can use to combine two monsters to finally be able to get Ghost. And that's just the second game.
Monster Rancher also has a mountain of hidden stats, much like Pokémon—such as monster lifespan, fatigue, and stress. While these are all numerical values, you can never see said numbers, and the game doesn't bother to tell you how they affect your monster—or how various items affect those stats. Because you almost have to micromanage your monster's stats, this can get extremely frustrating.
The game never gives any hint about the three strongest enemies in the game, to clarify, these 3 Bonus Boss are found by entering a specific Tournament that only appears once every several years, needs you to qualify for it by winning a tournament a month before that (this get explained in game and said match is often found by coincidence), and to face them you need to be Rank S and above. Then, after you can actualy enter the Tournament and fullfill the requirements, you need to Soft Reset until the game Randomly Drops said monster for you to fight, but the chance is really slim for meeting even one of those monsters. The closest you can get to a hint comes in Fridge Brilliance (it's a meetup battle beetwen two continents, so it's a normal thing to expect them to have powerful monsters that is at least on par with the Major 4 instead of the standard S class you usually meet).
Elder Cup. The game never implies or inform the existence of such a match, and unlike other special matches, the game doesn't even give confirmation message in form of in-game mail. The requirements? Have your monster to be at least 6 years and 3 months old and the Freshmen Cup will be changed into this. The monsters are very unlikely to live this long unless you rigourously follow a guide's training methods.
It used the raw numbers of the monsters as a base;
Then corrected them by multiplying by the level of gain for the stat;
Puts those stats in rank order 1-6 by tie-breaking;
Then compares the rank order between monsters to see how many of them match up;
The looks at the baby's stat baseline order to see how well it matches with the parents. Unless there is very high co-ordination betwen these elements, the monsters combined will simply not be that good.