Thrashers spawn both in Eriol (I found one at 36.4S, 38.7W) and NW of the Forsaken Inn (I found him at 31.8S, 38.6W), near where the barrels are for the goblin poisoning quest.
To get to the one in Eriol... Go up the set of stairs where the Wargs start to spawn, turn right, run past the round stone platform with the pillars on it and cross the bridge/go up the stairs straight ahead. At the top, there's a campfire. The Thrasher walks right past it.