LogFAQs > #979839005

Feebas in pokeymans Ruby and Sapphire. It had some absurd hidden rules governing when it would spawn involving invisible mechanics and even when you got it right, other pokeymans could still spawn in its place.

As an aside, to evolve it, you needed to max out its beauty stat, which was impossible if it had the wrong nature or something like that. So if you caught one that couldn't evolve, you'd have to catch another one or breed it (assuming it was female since there weren't wild ditto in the game).