This is just BAD ASS! You should totally make a whole set of these. I wouldn't mind at all seeing a lion duking it out with a giant scorpion, or an archer centaur putting holes in a large horned ram. Totally kickass.
One thing about this piece though: the gatling gut on the shark, while pretty awesome as it is, looks a little bit crooked and bent. It looks as if you might have drawn it without any photo reference, and it just looks a little bit rough and squiggly compared to the rest of the piece. I'd go over it again to make sure it makes sense with how it connects to that box thing.
Overall, and EXCELLENT piece of work! I love it. I don't know which of them looks fiercest, the bull or the shark, but I wouldn't wanna put money on who's gonna win.