Is it possible to add new environmental critters, such as different species of fish, to the game?

I've managed to duplicate some critters and set the dupes to use different textures so there's color variety, but I haven't been able to figure out how to introduce a new mesh and/or animation. I thought it might be possible to delete the mesh from the .nif file in NifSkope and just import the new mesh over the remaining animation skeleton, but that doesn't seem to work. Or at least I can't make it work. I have an animated skeleton and a static mesh. Can't seem to connect the two. While something like this, or at least re-using existing animations, would work for some critter varieties, there are some things I'm trying to add that would require a new animation, like a turtle. All the critters I'm contemplating would have very simple animations.

If someone can confirm adding new critters is possible, I'd appreciate it. Pointers to any tutorials on the subject would be great if you know of any non-video ones. Being deaf means most of them are lost on me, or take a lot more work to decipher what's going on than it's worth.