This is very easy to do.

The programs you need are:
Winrar
and Nifskope

Once you have both these programs open up Winrar and go to the ZT2 section.
Next, Find the animal you want to re-skin. (In my case the Allosaure) It won't work on most marine animals so I say you try a dinosaur first.
Double click on the animal file and you'll see a bunch of properties. Double click on the file called entities.
Next, go and double click units then animals. Now double click on the name of the animal and look for the name of your animal and it will apear a lot.

The file you want is the one with the the picture of the cube and the circle. Make sure on the file name it says nif. But don't open it if it says emote.

It will open up the Mesh in Nifscope and then look around on the left until you find files with a paint picture and then choose your colour.

Happy Re-Skining.