A pet was a creature kept by a sentient being for companionship.
Helen A had a pet Stigorax named Fifi. (TV: The Happiness Patrol)
Myfanwy was Torchwood Three's pet pterodactyl. (TV: Cyberwoman)
After running away from the Spacefleet Academy, Bernice Summerfield kept two pet turtles, Speckley and Squidgy. (AUDIO: The Shadow of the Scourge)
The Eleventh Doctor claimed that he did not keep pets, but made friends. (COMIC: After Life)