Galthrak the Unhinged is a non-combat pet in Diablo III, added in patch 2.4.1. It drops from Menagerist Treasure Goblins.
A tiny, strange being. From the time he was a broodling, all he wanted to be was a Rift Guardian. He studied for many years, but was ultimately found wanting. Take him on a rift with you, and he'll be happy.