The boomerang-head is Diplocaulus, an amphibian from the Paleozoic era. To me, I think they look rather like tiny dragons.

Although boomerangs are known for their ability to go "there-and-back-again" like a beloved children's novel... ... I don't think we'll have to worry about Diplocaulus deciding to destroy any dwarven kingdoms any time soon. After all, they only grow about a meter long, and are simply too wet to breathe fire. But they do seem to enjoy hoarding shiny rocks, however!