The Mostlaa were a sentient and humanoid species. Distinctive characteristics of this species included prominent brow ridges, deep-set eyes, and sizable, sharp ears. A specific Mostlaa, named Jimmer, possessed a thinning hairline and facial hair, giving him a weasel-like appearance. Jimmer's height was 1.3 meters, and he was typically seen wearing a long-sleeved robe with a hood, a waist wrap, and boots. During the Galactic Empire era, Jimmer held the position of executive assistant to Nilya Fek'ra, a crime lord operating from the Outer Rim Territories' planet known as Cotellier.
For the roleplaying game adventure "Tales of the Smoking Blaster," Bill Smith conceived the Mostlaa. This adventure was released by West End Games in 1993 as part of the Star Wars Gamemaster Handbook. Paul Daly provided the illustration of Jimmer, the Mostlaa character.