Animals That Start With X: X-ray Tetra, Xerus and More

X-ray tetra

The X-ray tetra is a small freshwater fish, at most about 4.5 cm (1.8 in) long, from the Amazon and Orinoco river systems of Brazil and Venezuela and coastal rivers of the Guianas. Its body is so translucent that you can see its backbone through its sides, which gives it its name. It is also called the X-ray fish and is a popular aquarium species.

Xami hairstreak

The xami hairstreak is a small butterfly of the family Lycaenidae, found from southern Arizona and Texas south to Guatemala. It has a wingspan of only about 2.4–2.9 cm (1–1.1 in) and thin, hair-like tails on its hind wings. The underside of each hindwing is yellowish green, crossed by a white line that forms a neat letter W near the tails. It is considered rare. Its caterpillars feed mainly on succulent plants, especially Echeveria and stonecrops (Sedum).

Xantus’s hummingbird

Xantus’s hummingbird lives only on the Baja California Peninsula of Mexico. It is 8.5–9.5 cm (3.3–3.7 in) long, and the male has a red bill with a black tip, a black face with a bold white stripe behind the eye and a rufous tail. It is named after John Xantus, a Hungarian zoologist who collected the first specimen, and it is listed as Least Concern.

Xantus’s murrelet

Xantus’s murrelet was the name of a small black-and-white seabird that nests on islands off California and Mexico. In 2012, ornithologists split it into two species, Scripps’s murrelet and Guadalupe murrelet, after finding differences in face pattern, bill shape, calls and genes. The chicks are remarkable: within two days of hatching they leave the nest and scramble down to the sea to join their parents.

Xenarthrans

Xenarthrans are a group of mammals of the Americas that includes the anteaters, tree sloths and armadillos, with 31 living species. The name means “strange joints”, after the extra joints between the vertebrae of the lower back, which strengthen the spine. Examples include the nine-banded armadillo and the giant anteater. Extinct relatives such as giant ground sloths weighed several tonnes.

Xenoceratops

Xenoceratops is a horned dinosaur, a relative of Triceratops, that lived in what is now Alberta, Canada, during the Late Cretaceous, more than 75 million years ago. Its name means “alien horned face”. It is known mainly from parts of its skull and bony neck frill, which had distinctive spikes, and it was formally named in 2012.

Xenopeltis (sunbeam snake)

Xenopeltis unicolor, the sunbeam snake, is a harmless, non-venomous burrowing snake of Southeast Asia. Its smooth, highly polished scales shimmer with rainbow colors in the light, which gives it its common name. It kills frogs, other reptiles, including snakes, and small mammals by constriction, and it lays up to about 10 eggs. It is listed as Least Concern.

Xenops

Xenops are small brown birds of the ovenbird family, with four species living in the tropical rainforests of Mexico, Central America and South America. They have a long tail and a flattened bill with an upturned tip. Xenops search for insects on bark, rotting stumps and thin twigs, moving nimbly in all directions along trunks and twigs, but without bracing on their tails as woodcreepers do.

Xenopus (clawed frogs)

Xenopus, meaning “strange foot”, is a genus of about 20 fully aquatic frogs from Africa south of the Sahara, named for the short black claws on their hind toes. They have no tongue, so the African clawed frog finds prey by smell and by sensing water movements, then uses its feet and claws to tear food and push it into its mouth. It was used in the first widely used pregnancy test in the 1930s and remains an important laboratory animal for studying how embryos develop. Experiments moving cell nuclei between Xenopus eggs and body cells helped the British biologist John Gurdon share the 2012 Nobel Prize in Physiology or Medicine. Released frogs are now invasive in several countries, including parts of Europe.

Xerus

Xerus is the scientific name of the unstriped ground squirrel, a reddish-brown squirrel of the dry savannas and scrub of East Africa, from Sudan and Ethiopia to Kenya and Tanzania. It digs burrows and has a long, flat tail; on average its head and body measure about 23 cm (9 in), with a tail of about 17 cm (7 in). The genus used to include other African ground squirrels, such as the Cape ground squirrel of southern Africa, which holds its bushy tail over its back like a parasol and often shares its burrows with meerkats.

Xoloitzcuintli

The Xoloitzcuintli, or Xolo, is a domestic dog breed from Mexico, best known as the Mexican hairless dog, though coated Xolos are born too. Its name joins Xolotl, an Aztec god, with itzcuintli, the Nahuatl word for dog. Ceramic figures of hairless dogs have been found in ancient burials in western Mexico, but a 2020 genetic study found that modern Xolos carry only about 3 percent of their ancestry from dogs that lived before the Spanish conquest.

Why so few animals start with X

Very few English words begin with X, and most of those come from Greek, where xenos means “strange” or “foreign” and xiphos means “sword”. In English, an X at the start of a word is usually pronounced like a Z, as in xylophone. Because everyday animal names grew from older English words, almost none of them start with X. Some lists online fill the gap with invented or misspelled names, so every name on this page belongs to a real animal, breed or animal group.

Scientists, however, use Greek roots all the time, so X appears much more often in scientific names. The swordfish, for example, is Xiphias gladius, which roughly means “sword sword” in Greek and Latin. For longer lists, see animals that start with W or animals that start with Y.