Creating new creatures.

Good afternoon. I have a question: I need to add a new creature to the game. I'm not a professional, and I don't need anything too complicated. I just want to slightly modify the model (helmet, sword, shield, etc.) and recolor the armor, skin, etc. That is, purely cosmetic changes. I don't need to change animations or skeletons, or anything else. But the creature should have a new ID and be separate, not replacing an existing one. Please tell me how I can do what I described above?