Eel bird

The Regulan eel bird was an animal native to the planet Regulus V. They are noted for their instinctive drive, every eleven years, to return to the caverns where they were hatched and mate. (TOS episode: "Amok Time")

A sub-species, the pygmy eel-bird, were noted for their shiny purple plumage, which reflected the UV rays of their planet's sun. A number of pygmy eel-birds were transplanted to planet Mestiko circa 2274, but their habitats were endangered by a massive forest fire in that year. (TOS eBook: Mere Anarchy: The Darkness Drops Again)