This tutorial gets you started with rotoscoping (masking) moving objects:
In AE CS5.5 I think you don't have the roto brush, yet. But the second workflow using mocha is still available in this old version. You don't have the Refine Edge Tool, but you also get decent masks with mocha alone.
Once you have a mask, copy it on your animal and set the mask mode to subtract such that the masked part is removed from the animal layer.
Wow, that was a neat tool, the roto brush I have successfully isolated the object now, and I am currently trying to convert the roto-selection into a mask, so I can apply it to the other layer. I didn't see them doing something like that in the video thoguh. I will keep searching for solutions online, but if you have any ideas I'd love to hear them
Hmm, the roto brush seems to give very rough edges. I think I'm better off just creating a mask, since I am working with very sharp and straight edges here.
It really depends on the shot. Sometimes Rotobrush is better, sometimes masks (probably best with mocha).
If you use the roto brush, you don't have a mask. But you can use the roto brush layer as a track matte layer for your animal.