I heard that you can put into game a new model of the monster, leaving the old animation. In a 3D program to edit it? Thank you.
I'm used to making new animations to prevent bugs as much as possible, but it may be possible if you take the old model's bones and import a new mesh. Save it as a new monster and your should be set. Of course, it's going to take a little more effort than that and time as well, and I haven't tried it but it's possible.