How to add a collision to a monster?

Well, your question is so vague it is hard to even begin. For starters, your monster will need a collider and depending on what behavior your monster will have it can vary. If you plan on this monster moving or having pathing of some sort, go ahead and throw a character controller on it. Make sure that it is positioned above the ground it is walking on (you do not want that monster clipping through the ground or it will fall through) and make sure your level it is walking on has some sort of collision as well. Pretty much for anything to move around and still be affected by gravity it needs to have colliders so start there.