Helpswapping player models with monster models correctly

When playing around with the hfs files I decided to try something new. I decided to swap the female_darkknight files to make a custom transformation mod. I chose a random enemy and it was everwhite so I copied the gnoll_everwhite files from an hfs file, renamed the copies to female_darkknight and swapped them with the original female_darkknight files in another hfs file. People experienced in this stuff should understand what I am talking about. Well after transforming ingame I got a weird Everwhite model with its mouth widely open and the animation "stuck". So i just closed Vindictus. Forgot about the changed hfs and started vindictus again later but got an error as soon as it loaded a dungeon which I believe was because of the messed up hfs file. So I changed the hfs file back to normal with the original darkknight files. Everything worked fine after that.

But now here's my question: Is there anything I did wrong or is a monster model completely useless as a player's model?

i think there are motion files and some other stuff inside the model files so it'll be all messed up if you just switch them like that...
it can be done (as you can see with the succubus trans mods) but it requires a lot more work than what you've done

^ That. Human models have the same animation so they wont cause errors but having a monster model for your player will result in it. You will have to edit motion files to fit in whatever the game has for human models which goes beyond hfs editing. Think more like a graphics designer, lol.

You can youtube the failed bear mod and see it, it's pretty hilarous yet interesting concept to explore nontheless.