Unlike the normal Mounting Animation(it technically is an animation) where the animation defaulted tot he M2 file without the need of ANIM Files.. The Mounting Animation is a custom animation that is read off of the ANIM Files, so if you are missing some ANIM files or don't have any at all, that's the issue.
Or if the ANIM Files you have on the New file did not belong to the original that you may have used before the mod, that's also where the issue may lie.