The nexu was a vicious, snarling, predatory, cute, cuddly, adorable kitty from some planet that nobody cares about. They were notable for their four eyes and two tails. They were used in many places throughout the galaxy as gladiator-beasts or as guardian monsters for an evil warlord's swamp headquarters. They were highly desirable because of their hunting prowess, their ability to see infrared light, and their ability to make little girls squeal with delight as they batted balls of yarn.
A nexu slashed at Padmé Amidala on Geonosis, but it merely left a sexy scratch-mark on her bare midriff. Better luck next time, nexu! Jango Fett fought a few of them in a video game once. Allana Solo kept one as a pet, awwwwwww.
Behind the scenes
The plural of "nexu" is "nexus," which is really confusing. It has nothing to do with Star Trek: Generations, however.