Hammer-Heads
Armless creatures with flat-topped heads that can shoot them forward as projectiles, guarding a hill against intruders.
First named in The Wonderful Wizard of Oz, ch. 22
Far away in the South Country there lived in the mountains a band of strange people called Hammer-Heads
As the books describe them
He was quite short and stout and had a big head, which was flat at the top and supported by a thick neck full of wrinkles. But he had no arms at all, and, seeing this, the Scarecrow did not fear that so helpless a creature could prevent them from climbing the hill. The Wonderful Wizard of Oz, ch. 22