From Chevy Wiki

The Chevrolet Bruin (and similar GMC (General Motors division) Brigadier) were Heavy-duty Truck made by General Motors between 1978 and 1988. The Bruin & Brigadier were both conventional rigs using the 9500 series cab introduced in 1966; this cab was better-finished and more stylized than a typical big-rig cab but larger than the pickup-based cab used on the contemporary C40-C65 series. Diesel engine choices included Caterpillar Inc., Cummins and Detroit Diesel powerplants.

"Bruin" followed the pattern of "frontier beast" names given to heavier Chevrolet trucks such as the Chevrolet Bison (heavier) and Chevrolet Kodiak (lighter). "Brigadier" followed the pattern of "military rank" names given to GMC's heavier trucks. Unlike the Titan/Astro & Bison/General, the Bruin/Brigadier found suitable replacements in the form of the Kodiak/Topkick

For some reason, the GMC version was more common than the Chevrolet version.