Another one of my 12-sprite projects, featuring Croconaw. As he didn't appear in Gen1, I did some looking and discovered that 7g [the little one] and LM4 [the big one] both appear as Croconaw when viewed in the trade center in gen2, though they cannot be traded over.

7g is a water/psychic glitch Pokémon that doesn't evolve normally, and only knows Confusion and 2 glitch moves. Only found in Yellow using a modified Mew trick/glitch.

LM4 is a water/fighting glitch Pokémon that, on hitting level 18, evolves into Clefairy and then instantly into Nidoking. Only found in Red and Blue using the same method.

Croconaw... you already know. Type Water, evolves from Totodile at level 18, evolves into Feraligatr at level 30.