I read this a zillion years ago but somehow lost or gave away my copy. It’s a 1954 book which appeared first in The Magazine of Fantasy & Science Fiction, slightly abridged. The Star Beast is one of Heinlein’s juveniles, and it’s great fun. It holds up pretty well 60 years on, at least in part because there’s very little real “science” in this book. It’s a pet-and-owner tale.