I'm trying to remember a story set on future earth, IIRC.
The alien is a baby-talking giant caterpillar-y kind of creature. a 'leave it to beaver' type boy looks after it, his dad maybe has military/nasa-type links hence why they have this alien. Its a 'Lassie' sort of story.
At the end the military is intervening, the pet's parent is rampaging, and suddenly the pet grows arms and picks up a rock to defend itself, thereby demonstrating the last checkpoint set by the space agency or whatever for 'sentient alien life', meaning it's murder to kill it.
This would have been in print probably in the 50s at the earliest but could have been added to whatever elementary school library between then and 1990