Take your pick:
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)
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I think the Xenopus toad is the best-known example of an animal beginning with X The toad has claws on its front and hind toes. The hind toes have webbing that helps with swimming. They are adapted to life in muddy waters and live in central and southern Africa. They eat smaller amphibians, fishes, arthropods and even carrion. They are a browny-green colour mottled with black on top and underneath are rather pale to almost white.