I couldn't find any common names beginning with x but there are quite a few scientific names begining with x. For example, Xenocongrid eels, Xiphonycteris spurrelli (a bat), and Xenicus longipes (NZ bush wren).
Xema are gulls, xenarthra are anteaters and xiphias are swordfish. So, there you have a creature from each of air, land and sea.
Quizmonster Mon 17/6/2002