In the May 1999 Kickshaws, Dave Morice said that he encountered (inprint) a Xantusiid lizard, and wondered whether there are any other X-animals. As an avid birdwatcher, I can answer his question for the feathered portion of the animal kingdom. Of course, I am assuming he is talking about X-birds in the onomatic sense and not in the sense that John Cleese used the term when complaining about his lifeless parrot.
"The X-Bird Files,"
Word Ways: Vol. 32
, Article 25.
Available at: http://digitalcommons.butler.edu/wordways/vol32/iss3/25