The Monster's Ring — Bruce Coville

MONSTROUS MAGIC
It all starts when Russell Crannaker buys a "monster's ring" from weird old Mr. Elives. Russell is sure the ring is just a silly magic trick -- but he follows the instructions anyway, and slips it on his finger. Then he whispers the magic chant, twists the ring once, and suddenly he's sprouting horns! Hair covers his face and hands. Russell has become ... a monster!

Russell quickly learns how to change back and forth from human to monster. But he hasn't paid enough attention to the directions, and when he puts on the ring on the night of a full moon, and twists it three times, Russell realizes he's gone too far. . . . A monster is loose in Kennituck Falls.