Undertale TF RP

So we came to a mountain and we fell into a hole (just like in the game) after the fall we walked around for a bit and later on we transform slowly into a monster either an enemy or one of the main monsters like Papyrus, Sans, Toriel, Asgore, Asriel, Muffet, Undyne, Mettaton, etc. Though I may need help on certain monsters since I know the game but not all of the enemies. So please provide an image of an enemy so I'm able to do the changes or if you're doing your own change.