A minotaur with balls bigger than a human head that just wants to rut, without regards to species or actually being able to reproduce. Animal desires.
A lizard man, dragon, or naga that wants to play with its food. Human intelligence but inhuman motivation.
A slime, jellyfish, or fungus that invades it's prey with tentacles/tendrils without actually being conscious at all. Rape as a naturally evolved tool for subduing prey.