The Star Beast (1954) – John Thomas Stuart XI has an alien animal as pet but after hundreds of years in the family, she has started to grow and is wreaking havoc in their small town. The local authorities want her destroyed. Havoc, too, is gathering in interplanetary space, where a powerful alien race has come to rescue their kidnaped princess or destroy the Earth.
For more information on Heinlien's Juveniles (written between 1947 and 1958) see our entry on Robert Heinlein's Rocket Ship Galileo, The Literary Encyclopedia profile by William Patterson, University of California, Santa Cruz
Patterson, William. "The Star Beast". The Literary Encyclopedia. First published 10 November 2004
[https://www.litencyc.com/php/sworks.php?rec=true&UID=11024, accessed 22 November 2017.]