How do I add a new mount

[uanime5]

I plan to alter the elephant, however since it is a mounted unit simply adding the new meshes to battlemodel file doesn't work. I've tried adding this new mount to descr_model_strat.txt and descr_mounted_engines.txt but the game keeps throwing up error messages saying that this mount needs a unique identification code. Does anyone know how to add a new mounted unit to the game?