Giant creatures of Regulus V that return every eleven years to the caverns where they hatched, cited by Spock as a parallel to the Vulcan mating drive.
The giant eel-birds of Regulus V are creatures governed by a powerful homing instinct. Every eleven years they must return to the caverns in which they hatched.
They are mentioned in Amok Time, when Spock is at last forced to explain to Captain Kirk why he must return to Vulcan. Struggling with a subject Vulcans do not discuss, he reaches for examples from nature: the eel-birds of Regulus V and the salmon of Earth, which swim back to the stream where they were born to spawn or die in the attempt.
The comparison lets Spock describe the pon farr as a biological compulsion rather than a failure of logic. The eel-birds themselves are never seen.