The Star Beast by Robert A. Heinlein

The Star Beast is a slightly more "mature" young adult novel by Heinlein. John Thomas is the owner of an extraterrestrial beast known as Lummox, who has been passed down from generation to generation - the creature was adopted by his grandfather on a voyage of exploration beyond our solar system, and has been living with the family ever since. Lummox is about the size of a rhinoceros, with armor plated hide and an appetite like a goat - eat anything that's not tied down. One day Lummox decides that the neighbor's rose bushes look quite appetizing, and ends up causing thousands of dollars in damages after being chased off by the neighbor, attacked with a shotgun by another, and pursued to the edge of town by the police.

The ensuing civil/criminal trial of John Thomas comes to the attention of the bureau of Extraterrestrial affairs, and the undersecretary sends his assistant to find out if the ET in question is a member of any of the known intelligent races in the galaxy. Lummox doesn't at first fit the criteria for recognition as a "human" equivalent being, as it doesn't have any hands. When the court decides that Lummox should be destroyed as a dangerous nuisance, John Thomas and Lummox run away to hide.

It turns out, however, that 1) Lummox is a girl, 2) she is the culmination of a millenia long breeding project by the Hrotha, who have come to Earth finally to recover their prodigal daughter, and 3) has been slowly maturing over the years and grows a set of arms and hands while she and John Thomas are wandering in the wilderness.

The story, however, is not so much about the two main characters as it is an an educational yarn about how government bureaucracies and diplomatic efforts work. Heinlein had plenty of experience dealing with the government when he left the Navy to work for a defense contractor, and he shows us in this story where the real powers in government live.

One of Heinlein's more enjoyable YA tales.