The Mountain nerf was a subspecies of nerf that was brought to the planet Fennesa. They were descended from nerfs which had been introduced there thousands of years ago and had adapted to the mountainous conditions there. Mountain nerfs tended to be nimbler than the varieties living on flatter terrain. The Fennesa Nerf Herding Council collected data regarding nerf fatalities from various variables on the rocky planet.
Sources
Creature Feature: Mountain Nerf on Wizards.com (original site is defunct)- The New Essential Guide to Alien Species
- The Complete Star Wars Encyclopedia (as Fennesa mountain nerf)
