Swan

Swans were birds native to Earth. They mated for life and often appeared on greetings cards. (TV: Something Borrowed)

Swans were occasionally eaten. In 1400, Vicki and Isabella heard one being killed in preparation for Epiphany. (AUDIO: The Doctor's Tale)

In Greek mythology, Castor and Pollux were born when Zeus took the form of a swan and seduced Leda, their mother. The Tenth Doctor thought that was why, on the colony world of Aractus, they still said, "Never turn your back on a swan". (PROSE: Wooden Heart) Achilles reminded the First Doctor (whom he believed to be Zeus) that he appeared to Leda in the form of a swan. (TV: The Myth Makers)