The Star Beast by Robert Heinlein
Paperback. Condition. Good/Good+. 1954. ACE. 1st Edition.
Part of a series of books Heinlein wrote for children.
Lummox had been the Stuart family pet for years. Though far from cuddly and rather large, it had always been obedient and docile. Except, that is, for the time it had eaten the secondhand Buick . . .
But now, all of a sudden and without explanation, Lummox had begun chomping down on a variety of things—not least, a very mean dog and a cage of virtually indestructible steel. Incredible!
John Thomas and Lummox were soon in awfully hot water, and they didn't know how to get out. And neither one really understood just how bad things were—or how bad the situation could get—until some space voyagers appeared and turned a far-from-ordinary family problem into an extraordinary confrontation.