Count: 0. There are no extant, commonly accepted mammal species whose English common name begins with the letter X. This list follows authoritative taxonomies (IUCN Red List, Mammal Species of the World, GBIF, Encyclopedia of Life) and limits entries to widely used English common names; under those criteria, the letter X yields no results. Interestingly, X does appear in scientific and higher‑level names — for example the superorder Xenarthra and genera such as Xerus and Xenomys — so X is present in technical taxonomy even though it rarely starts everyday English common names for mammals.

Recognize that English vocabulary and common naming practices rarely produce words that begin with X, and taxonomic naming conventions more often place X at the start of Latinized genus or family names rather than in popular common names. Rely on searches of the major databases (IUCN, Mammal Species of the World, GBIF, EoL) to confirm the absence of accepted English common‑name mammals beginning with X. Consider close matches instead: several extant mammals have scientific names or genera that begin with X (for example Xenomys and Xerus), and regional or transliterated local names in other languages sometimes start with X but do not meet the requirement for a widely accepted English common name. Consult lists organized by scientific name or by higher taxa (such as Xenarthra) if you want entries that begin with X in formal taxonomy.