Model looks weird when play animations

Hello everyone, I’m new both here and on UE4, so this may seem like a dumb problem but I have extracted some animations and game model from elsewhere. Now while the model looks good enough I really can’t understand why the animations makes the model looks like this https://i.imgur.com/445m6NL.png when I play it, while the normal model looks like this https://i.imgur.com/1fJnI5G.png

Anyone knows what’s happening or why?