MGS4 had fewer animals and insects than MGS3. Here are some that I found tucked inside the game photo area from a while ago. In all there were about 8 different types of lifeforms. Also, there are 4 different types of Guinea pigs (color-wise). So in all, if you count different colors, there are 11 different kinds of animals and insects. If you know of any more feel free to post some pics. Sadly, there were no fishes. Note that if you count cut-scenes there was also a snake that swallowed a guinea pig, a centepede and a monkey, but I couldn't find them in-game, so I didn't count them.
To start off, here are some wolves:
