14 Animals That Start with X, from Cretaceous Sea Monsters to Aztec Dogs
October 25, 2026
Animals that start with “X” are very rare in English. That scarcity is part of the fun. Almost every name comes from Greek, from a collector called John Xantus, or from a river in Brazil. A good share belong to creatures that disappeared millions of years ago.
This list of animals that start with X covers 14 of them, from a see-through aquarium fish to a Cretaceous predator dug out of Kansas chalk. Some sit in American pet stores, some in museum drawers, and one in Aztec myth. Anyone working through the whole alphabet can pair it with the page on animals that start with W.
One note on the last entry. The Cape ground squirrel belongs here because its genus name, Xerus, begins with X, and the animal is usually filed under that name.
1. X-Ray Fish
The X-ray fish (Pristella maxillaris) is a small South American tetra with a body so clear that its spine and swim bladder show through the skin. HowStuffWorks lists the species under that name, and you can find it on that site’s list of X animals. In the wild it lives in the coastal rivers and slow streams of northern South America, including Guyana, Suriname, Venezuela, and the Amazon basin.
Adults reach only about two inches. The fins carry bands of yellow, black, and white, which stand out against the glassy body. That contrast is why the fish became popular so quickly with hobbyists who wanted something unusual but easy to keep.
2. X-Ray Tetra
The X-ray tetra is the same animal as the X-ray fish, sold under its more common trade name. It has been a staple of American fish stores for decades because it is hardy, peaceful, and cheap. Hobbyists also call it the golden pristella tetra or the water goldfinch.
For anyone planning a first community tank, the basics are simple:
- Group size: keep at least six, since lone fish turn shy and pale.
- Water: warm, soft to moderately hard water, roughly 72 to 82°F, suits them well.
- Tank mates: pair them with other small, calm fish and avoid fin nippers.
- Food: flakes, micro pellets, and frozen daphnia or brine shrimp all work.
Pro Tip: Compare the X-ray tetra to the neon tetra, a longtime tank favorite. The animals that start with N page covers that fish, and the two species share similar water needs.
3. Xantus’s Hummingbird
Xantus’s hummingbird (Basilinna xantusii) is a small, green-backed hummingbird with a bold white stripe behind the eye and an orange-red bill. It is native to the Baja peninsula of Mexico, and Petnicki’s quick-reference table in its X animals roundup places it in Baja California. Sightings in the United States are extraordinarily rare.
The name honors John Xantus, a Hungarian-born naturalist who collected specimens in Baja California in the 1850s and 1860s for the Smithsonian Institution. His legacy runs through several animals. The night lizard genus Xantusia is also named after him. Birders who enjoy odd names can continue with bird names that start with K.
4. Xenacanthus
Xenacanthus was a freshwater shark that swam in Paleozoic rivers and swamps roughly 300 million years ago. Its name means “strange spine,” a nod to the long, sharp spine that rose from the back of its head. The body was long and eel-like, and the animal probably reached about three feet.
Unlike most sharks, xenacanths lived in fresh water. Fossils of the genus come mainly from Europe, while relatives such as Orthacanthus turned up in the Permian red beds of Texas and Oklahoma. Fossil hunters often find the spines and teeth first, since shark skeletons are made of cartilage and rarely survive intact. If sharks interest you, the animals that start with S list has plenty of living ones.
5. Xenoceratops
Xenoceratops foremostensis was a horned dinosaur that lived in what is now southern Alberta, Canada, about 78 million years ago. HowStuffWorks includes it under that full name among its X animals. The genus name means “alien horned face,” and the skull carried a frill with long spikes along its edge.
Its story is as interesting as its looks. The bones were collected in 1958 by paleontologist Wann Langston Jr., then sat in museum storage for decades. Researchers described the animal in 2012, and it counts among the oldest large-bodied horned dinosaurs known from the region. It was a distant relative of Triceratops, whose fossils are well known across Montana and Wyoming. Fans of Cretaceous giants can explore the animals that start with T page.
6. Xenoposeidon
Xenoposeidon proneneukos is a long-necked sauropod known from just one partial vertebra. The bone was found near Hastings, England, in the 1890s and sat in the Natural History Museum in London for more than a century. Paleontologists Mike Taylor and Darren Naish gave it a name in 2007, which means roughly “foreign Poseidon.”
The vertebra looked wrong for any known sauropod, with a tall, forward-leaning neural arch that is unusual for the group. Later work by Taylor suggested the animal belonged to the rebbachisaurids, a branch of sauropods. That makes it an example of how a single bone can reshape a family tree. For a more familiar relative, see the animals that start with D page and its Diplodocus.
7. Xenops
Xenops are small, brown birds in the ovenbird family that live in the forests of Central and South America. The genus name Xenops comes from Greek words for “strange face,” and the birds have a short, slightly upturned bill. The streaked xenops (Xenops rutilans) is the species most often named in lists, and HowStuffWorks includes it.
These birds forage like chickadees, hopping along branches and sometimes hanging upside down to pry insects out of bark and rotten wood. Most checklists recognize four species. Some online lists say three, so counts can vary by source. American travelers sometimes spot xenops on birding trips to Costa Rica, Panama, and Brazil.
8. Xenopus
Xenopus laevis, the African clawed frog, is the best-known member of a genus of fully aquatic frogs native to sub-Saharan Africa. These animals have flat bodies, no tongue, and three short claws on each hind foot. The genus has a rich history in scientific research and pregnancy testing, as the Frosting and Glue animal list notes.
The frog’s path through science runs roughly like this:
- 1930s: Biologist Lancelot Hogben shows that injecting a pregnant woman’s urine into a female frog triggers egg laying.
- Mid-century: Hospitals and labs across the world, including those in the United States, keep colonies of the frogs for routine testing.
- 1960s: Chemical tests replace the frog method.
- 1962: John Gurdon clones frogs from adult cell nuclei, work that earned a share of the 2012 Nobel Prize in Physiology or Medicine.
- Today: Xenopus remains a standard animal in developmental biology.
The popularity of the frog had a downside. Escaped or released animals formed feral populations in places such as California, Arizona, and Florida, where they compete with native amphibians. Researchers have also flagged them as possible carriers of chytrid fungus.
Pro Tip: Never release a pet clawed frog outdoors. Several states restrict ownership, so check with your state wildlife agency before buying one.
9. Xingu River Ray
The Xingu River ray (Potamotrygon leopoldi) is a freshwater stingray found only in the Xingu River, a major tributary of the Amazon in Brazil. HowStuffWorks pairs the common name with that scientific name in its list. The ray has a dark, almost black disc dotted with white or cream spots, a pattern that looks like a night sky.
Its tiny range makes it vulnerable. Dams on the Xingu, including the Belo Monte complex, have altered the river’s flow, and collecting for the aquarium trade adds pressure on wild populations. Large public aquariums in the US sometimes keep river rays of the same family. Rays are also a good reason to visit the animals that start with R page.
10. Xiaotingia
Xiaotingia zhengi was a small, feathered dinosaur from the Late Jurassic of Liaoning Province, China, about 160 million years ago. Paleontologist Xu Xing and colleagues described it in 2011. The genus name honors Zheng Xiaoting, a curator at the Shandong Tianyu Museum of Nature, and the animal measured about half a meter (roughly 20 inches) long.
Its fame comes from what it did to Archaeopteryx. Xu’s team ran an analysis that moved Archaeopteryx away from the earliest birds and into a group of bird-like dinosaurs, shaking a textbook case. Later studies pushed back, and scientists still debate where Archaeopteryx belongs.
Important Note: Xiaotingia’s position in the dinosaur family tree is contested. Different anatomical analyses place it and Archaeopteryx in different spots, so treat any single diagram as a hypothesis rather than a final answer.
11. Xiongguanlong
Xiongguanlong baimoensis was an early tyrannosauroid that lived in northwestern China during the Early Cretaceous, more than 100 million years ago. Researchers described it in 2010 from a skull and partial skeleton found in Gansu Province. The name means “Xiongguan dragon,” after a historic pass on the Silk Road.
Unlike the deep-skulled Tyrannosaurus rex, Xiongguanlong had a long, narrow snout and a body around 16 feet long. It fills a gap between small, early tyrant dinosaurs and the huge ones that came later. In the United States, a related find came from Utah, where Moros intrepidus was described in 2019 as a small tyrannosaur that lived about 96 million years ago.
12. Xiphactinus
Xiphactinus audax terrorized the inland seaways of what is now Kansas about 83 million years ago, reaching up to 15 to 20 feet long. The fish looked like a gargantuan, fanged tarpon. It hunted in the Western Interior Seaway, which covered the middle of North America during the Late Cretaceous.
Its most famous fossil is the “fish-within-a-fish.” The specimen is a 13-foot Xiphactinus with a nearly perfect, six-foot Gillicus arcuatus inside it. It was recovered from Gove County, Kansas, in 1952 by George F. Sternberg, the paleontologist from Kansas best known for the find. The fossil now sits at the Sternberg Museum of Natural History in Hays, Kansas.
The large fish apparently died soon after its meal. The likely cause was the smaller fish’s struggling, which may have ruptured an organ as it was swallowed. Joseph Leidy named Xiphactinus in 1870, and for years its fossils were mixed up with specimens E. D. Cope called Portheus molossus. FossilEra’s overview of Xiphactinus offers an accessible account of the fish and the Kansas chalk beds where it turns up.
13. Xoloitzcuintli
The Xoloitzcuintli, usually shortened to Xolo, is a mostly hairless dog from Mexico with a very long history. Orvis says the breed developed 3,500 years or more ago, and its breed history page ties the name to Xolotl, an Aztec god. The Xoloitzcuintli Club of America points to clay and ceramic statues of hairless dogs, more than 3,000 years old, found in Mayan, Colima, and Aztec tombs. Many people regard it as one of the oldest dog breeds in the world.
Its story in the United States has had a few turns. Xolos were among the first breeds the American Kennel Club recorded, and a dog named “Mee Too” was the first registered Xolo in 1887. The AKC dropped the breed from its stud book in 1959 because it was so scarce. A Mexican revival effort followed. The “Xolo Expedition of 1954,” led by Norman Pelham Wright, found enough good dogs to form the foundation of Mexico’s program to revive the breed. It is now the “Official Dog of Mexico.”
Important Note: Sources disagree on when the AKC took the breed back. Wikipedia says the AKC voted to readmit it to its Miscellaneous Class starting January 1, 2009, while Orvis reports 2010 for that step and 2011 for the move into the Non-Sporting Group. All of them agree on the 2011 Non-Sporting date.
Today the AKC accepts all three sizes under one breed standard. Hairless Xolos are the classic look, but a coated variety also exists.
| Size | Height at Shoulder | Typical Weight |
|---|---|---|
| Toy | 10 to 14 inches | 10 to 15 pounds |
| Miniature | 14 to 18 inches | 15 to 30 pounds |
| Standard | 18 to 23 inches | 30 to 55 pounds |
Heights are from Modern Dog Magazine’s breed profile, and weights come from Breed Advisor’s guide. Bare skin burns, so Xolos in sunny states like Arizona and Texas often need sunscreen in summer and a coat in winter. More dogs with unusual backgrounds appear on the dog breeds that start with R page. Readers curious about other ancient lineages can try horse breeds that start with B, which includes the Barb, a North African breed with a long history.
14. Cape Ground Squirrel
The Cape ground squirrel (Xerus inauris) lives in the dry grasslands, scrub, and semi-desert of southern Africa, including South Africa, Namibia, and Botswana. The genus name Xerus comes from the Greek word for “dry.” Petnicki’s X animal table lists the xerus as an African mammal, and HowStuffWorks uses the South African ground squirrel name for the same species.
The animal’s best-known habit is using its bushy tail as a parasol. When the midday sun is fierce, a squirrel will flip the tail forward over its back and stand in the shade it creates. It weighs only about a pound and digs burrows in firm, sandy soil.
These squirrels are social and live in groups of related females with their young. They often share burrow systems with yellow mongooses and suricates, and the three species watch for predators together. They eat roots, bulbs, seeds, and grass.
That wraps up the full run of animals that start with X, from an alien-faced dinosaur to a tail-shaded squirrel. The alphabet series continues with the animals that start with Y page.