A sprite. BUT NOT JUST ANY SPRITE. It's a combination of Seaking, Poliwag, and, you guessed it, MANTINE. Incredible, isn't it?
It was originally for Cooliocool
's sprite contest (Water Pokemon Splicing), but I had to come up with something to be accepted, and then the contest ended... SOOOOO now this is something I put together to be eligible to enter in future sprite contests by Coolio. For more info on which one I'm talking about, click here.
Sprites... Well I guess you don't really HAVE to credit anyone, because they're so widely used and everyone has some version of the same picture, but I believe they are from either The Spriters Resource
or some Pokemon Fansite I can't remember at the moment. Whoever you are, thanks a lot :)