Pokemon fit into five groups mainly: Nonbreedable, fish, birds, reptiles, and platypi/echidnas. (Only two mammals that are capable of laying eggs)
In the Pokemon: the Movie 2000 short, Poliwhirl says 'Poliwrath' multiple times during the bellossom scene.
It is a common misconception that Rock Pokémon are immune to electricity, but in fact Electric attacks deal neutral damage to Rock-types.
Dragin ond Ghost are weak to themselves.
In Red, Blue and Yellow, there was, however, only one Dragon move, Dragon Rage, which deals set damage so in fact, Dragon's weakness to Dragon never actually came into account.
Many people think that Waterfall was a new move in G/S/C, but it existed in R/B/Y - it was just learned only by Goldeen and Seaking, Pokémon that nobody uses, cares about or trains to level 39 (when Seaking learned Waterfall).
Bulbasaur was originally green through-and-through, but he was turned blue-skinned in third gen. His shiny form and sprite are still completely green, however~