Piranhas. They're so underappreciated in modern games, aren't they? Honestly, I don't think I could name a game that's come out recently that's utilized piranhas well whatsoever. That's where you guys come in! Know any cool games featuring our many-teethed friends?
Deadstorm Pirates (PS Move) has schools of piranhas that jump in and out of the water then attack your face, good game by the way.
Also, creatures formed out of leaves. This doesn't work for piranhas obviously, but you gotta love when a gust of wind kicks up a bunch of fall leaves and they swirl around, eventually taking the shape of a boar or eagle or whatever. It's like, "fuck, how do I defeat this!? My sword passes right through it!"
I gave "Piranhas in video games" a quick google search and could only find this... http://www.giantbomb.com/petey-piranha/94-1805/ And it's not even a piranha, as someone else already mentioned.