Here is a sculpt of the giant fish from Skylanders. He hangs like a taxidermy fish at my desk at work :) This video shows the process that I used:
I used aluminum foil to shape the general shape and size first. This is to help the Sculpey clay to have an even thickness, otherwise the inside of the fish will never cook before the outside is fully baked. Once I had a base, I just sculpted the fish, adding more detail as I went along. The teeth are made from Shapelock, as small sculpey teeth would break easily.
I then baked the fish, and used Shapelock under the fins to help support them. I painted the fish with acrylic paint.... the cheap stuff that's made for crafts/kids, which are thicker than model acrylic paint. Plus they are super cheap, a dozen colors for a few bucks.
It was fun making this thing. The Leviathan fish was something that I had designed and modeled for Skylanders, but you only see him for a second in the game, so it's nice to show him off as a trophy fish :)