Imaginative, unusual alien lifeforms, not typical bipeds which we generalize as quintessential extraterrestrial beings.  Complete with clothing.  Why do we assume aliens are naked aggressors with the technology to travel the stars, yet do not grasp the concept of decent outfits. 

Alien, or unusual, flora and fauna.  Creatures of fantasy and horror. 

Just about anything that doesn't have a shred of identifiable humanity embedded in it.