Haven't found a mammal but have found Xantus Hummingbird and Xantus Murrelet - a small black and white seabird a bit like a tiny penguin. A xanthareel, a yellow eel formerly used in medicinal foods, xiphias, a swordfish, xiphosuran, a horseshoe crab. So... don't let the lack of "X" mammals induce xerostomia or xerophthalmia caused by searching web sites, which, in turn could lead to xerodermia or worse yet, xenophobia...