Saw someone asking about evolution and creature design the other day somewhere on CA which got me thinking about my previous academic occupation, Biology =). So here's a big ol' list of wikipedia links of various topics dealing with critters.
A lot of the wiki articles will have multiple subtopics/links on them and usually some general examples associated with the classification for ease of use.
I found a most excellent fish skeleton reference page. There's a huge variety, and the images are very high quality. There may even be skeletons of other kinds of animals on this site, but I haven't explored beyond the fish page yet.