Many of the Pokémon we know (and probably many future ones) are based on animals. With 4 generations, a lot of animals have been used as a basis for a Pokémon. Some several times.
Do you have any ideas which animals haven't been used (yet)? I couldn't think of many. So, looking for animals that don't have a Pokémon based on them. Post your thoughts.
Actually, throw in plants too.
This thread isn't about new ideas for Pokémon, that one already exists somewhere.