Xenopus toad or clawed toad (Xenopus laevis)
Xantusiidae (famly of night lizards)
Xenarthra (suborder of mammals including anteaters pangolins etc.)
Xenicidae (family of New Zealand wrens)
Xenopeltidae (family of reptiles including sunbeam snake - Xenopeltis unicolor)
Xenipirostris (genus of birds from Madagascar)
Xenopterygii (order of fishes including clingfishes)
Courtesy of the good folks at:
http://www.thewildones.org/Scientists/askPhil.html #xanimals
For
a list of fish with common names beginning with Z see:
http://www.fishbase.org/ListByLetter/CommonNamesX. htm
i think there is something called a xanapus! it was on a preschool programme called animal alphabet! don't ask me what sort of animal it was though! i just remember the song that went with it!