Well, we have Raikou. It is essentially a tiger, isn't it?
I thought of a few other animals that are not pokemon yet:
Aardvark - I can easily see this being Ground / Poison; invading the homes of bug pokemon during the night and shooting sludge bombs at them through its snout. Possible name: Aardgunk (aardvark + gunk)
Okapi - As it is kind of a strange animal, I imagine it being a bulky Ghost / Normal (similar to Girafarig). Possible name: Ipako (okapi backwards_.
Chimaera (Ghost Shark) - Obviously a Ghost / Water type, but unlike Jellicent, those will be speedy glass cannons. Possible name: Chimaraith (chiaera + wraith)
COCKROACHES - Seriously, how are these not pokemon yet? I imagine them as Bug / Dark with fast, bulky and strong on the special side, but not so much on the physical side. They will have a signature ability called "Dreadful" which will be like Intimidate but lowers the special defense of the opponent (because he is scared / disgusted). Possible name - Feroach (feral + roach).
Mantis Shrimp - A Water / Fighting type with amazing speed, attack and special defense. It should have Moxie. Possible name: Stomarimp (stomatopod +shrimp).
We actually do have a mantis shrimp now. It may not completely look like it, but it does seem to be very close to it in theme.
Two headed turtle- A regular turtle that evolves into a two headed turtle. This may seem lame until you think about the fact that this is actually a relatively common defect in some species of turtles... can't remember which though. Perhaps it evolves from a mono water type into a dual water/poison or dark type?
Stink Bug or a Bombardier Beetle?
Daddy long legs?
An actual porcupine?
Perhaps a few cryptids (regardless of how real) such as that one really fast moving thing called a rod?