Any paranormal monster. So vampires, werewolves, ghouls, ghosts, etc. It can be from any origin so if you're wanting to use another culture's, like Japanese, you can have kitsunes, tengus, etc.
The jam is meant for visual novel styled games. I assume most people are going to go for the dating sim route, where you date one or more featured monsters, but it doesn't have to be a dating sim.