Garm is a large monster that is weak to Fire. It drops Brinebeast Ore, Brinebeast Fragment, Sinister Fang, Monster Fang, Aged Beast-Steak, Beast-Steak, Savagery Extract, Hunk of Rift Crystal, Wakestone Shard and Pointed Fang. In a fight you can also pilfer Wakestone from it, along with several of those drops.
The map has 7 Garm spots, spread singly around Ancestral Chamber, Bakbattahl, Battahl Ropeway Station Four, Debris-Strewn Coast and Rocky Lair. The Unmoored World has 7 spots of its own, with 3 around Bakbattahl and 1 each near the Eastern Forest and Harve Village, so Bakbattahl is the place to start a hunt there.