When the game crashes, it virtually always is a problem with editing either the nif or the bfm files. Make very sure the names are exact, and in the nif remember not to change the number of characters. You may have to shorten or lengthen the name of your animal in the nif. When you do, don't forget to use that exact name as the name of the dds file. For example, a gorilla to tiger would have to be something like gorila. A tiger to gorilla would have to become something like tigerxx. Then the dds would have to become tigerxx_M.dds.
Check the names in your nif file first, as that is most usually the problem.