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Devils Hole pupfish -

The Devils Hole pupfish is the rarest fish in the world. It's found only in Devils Hole, a water-filled cavern in Death Valley National Park in Nevada. The International Union for Conservation of Nature (IUCN) Red List designates this scarce wonder as Critically Endangered.

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Komodo dragon -

The Komodo dragon is the largest, heaviest lizard in the world—and one of the few with a venomous bite. The reptile is endemic to the Indonesian islands of Komodo, Rinca, Flores, and Gili Motang. It's an endangered species, according to the IUCN.

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Western diamondback rattlesnake -

Endemic to the southwestern United States and Mexico, the western diamondback rattlesnake has one of the most deadly bites in the world.

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Kemp's ridley sea turtle -

Listed as Critically Endangered by the IUCN, Kemp's ridley sea turtle is the rarest species of sea turtle in the world. The Gulf of Mexico and the Western Gulf waters of Texas are its last remaining strongholds.

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Giant oceanic manta ray -

The largest ray species, and one of the largest fish in the world, the majestic giant oceanic manta ray enjoys a global range in tropical and temperate waters. It's considered Endangered by the IUCN.

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Panther chameleon - A showstopping superstar, the panther chameleon is one of the most colorful of all chameleons. A native of Madagascar, it's coated with an array of colors that rival a rainbow.

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Coconut crab -

The largest terrestrial arthropod in the world, the mighty coconut crab is a huge land-dwelling crustacean with a taste for carrion, hence its nickname the "robber crab." Listed as Vulnerable by the IUCN, it's found on islands across the Indian Ocean, and parts of the Pacific Ocean.

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Saltwater crocodile -

This fearsome beast native to Southeast Asia, northern Australia, and Micronesia is the world's largest living reptile. Fully grown, males can reach 7 m (23 ft).

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Bulldog ant -

This is the most dangerous ant species in the world, a fact acknowledged by Guinness World Records. When attacking, the bulldog ant (also known as the bull ant) uses its sting and jaws simultaneously. They're found in Australia and New Caledonia.

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Golden poison frog -

Despite its diminutive size, the golden poison frog is likely the most toxic animal on the planet. Listed as Endangered by the IUCN, this deadly amphibian is native to the rain forests of Colombia.

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White-lipped island pit viper -

A venomous pit viper species found in eastern Java and the Lesser Sunda Islands of Indonesia, this white-lipped serpent can be bluish in color but is also seen in green, blue-green, and even yellow variants.

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Vietnamese mossy frog -

A master of camouflage, the Vietnamese mossy frog is also difficult to discern in south-central Laos and southern China.

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Whale shark - This denizen of the deep is the largest living nonmammalian vertebrate on the planet. Whale sharks are found in both coastal and oceanic habitats and feed almost exclusively on plankton and small fish. They pose no threat to humans. They are currently listed as Endangered by the IUCN.

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Rhinoceros viper -

Another master of disguise, this venomous viper species owes its intimidating name to the prominent nasal "horns"—two or three pairs of hornlike scales— set on the tip of the snout. Native to West and Central Africa, the rhinoceros viper is classified as Vulnerable by the IUCN.

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Frilled shark -

Looking like something out of a horror movie, the bizarre-looking frilled shark usually lives in waters of a depth of 600 m (1,968 ft). Its body shape and the number of gills are similar to fossils of sharks that lived 350,000,000 years ago. They are rarely seen in shallow waters.

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Satanic leaf-tailed gecko -

This weird-looking reptile calls Madagascar home. Its horned head, flaming red eyes, and a body that mimics dead leaves lends it a fiendish appearance, though in fact the satanic leaf-tailed gecko is shy and quite harmless.

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Purple harlequin toad -

This beautiful and charismatic toad species is seen in orange, green, yellow, brown, black, and purple coats, which they wear with aplomb in the rain forests of Central and South America. The purple harlequin toad is endemic to French Guiana.

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Hercules beetle -

An oversized species of rhinoceros beetle native to Central and South America, this humongous bug is the longest beetle in the world, and is also one of the largest flying insects on the planet.

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Plumed basilisk -

Native to Central America, this handsome species of lizard is distinguished by a crest on the back and tail, as well as large plumes on top of their heads.

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Puffer fish -

When threatened, puffer fish inflate themselves into a ball shape to evade predators. By doing so, they expose a skin of large externals spines, most of which are toxic. In fact, some puffer fish species rank among the most poisonous vertebrates in the world. They are found across the world in warm, tropical waters.

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Velvet ant -

Not an ant at all but in fact a wasp, this attractive critter is so named for its dense pile of hair, which most often is bright scarlet or orange. The insects are found worldwide.

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Red lionfish -

Striking in looks, the red lionfish is nonetheless a predatory member of the scorpionfish family. Native to the reefs and rocky crevices of the Indo-Pacific, it can deliver a painful sting, though its venom is rarely fatal to humans.

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Mombacho salamander -

Endemic to Nicaragua, the Mombacho salamander is named for its habitat, the high-lying slopes of the country's Mombacho stratovolcano. It remains Vulnerable on the IUCN Red List.

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Fiji crested Iguana -

Native to a scattering of the northwestern islands of the Fijian archipelago, this super rare species of iguana numbers just around 13,000 individuals. It's therefore designated Critically Endangered, according to the IUCN.

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Ocean sunfish -

One of the most peculiar-looking creatures on the planet, the ocean sunfish is a gentle saucer-like giant. Native to tropical and temperate waters around the world, it resembles a big floating blob and is the heaviest bony fish in the world. The IUCN lists this species as Vulnerable.

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Cane toad -

A large, warty, and poisonous amphibian native to South and Central America, cane toads average 10–15 cm (4–6 in) in length. Their diet is eclectic: they'll feast on small birds and mammals, and other reptiles and amphibians.

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Karner blue butterfly -

Dainty and scarce, this butterfly is named for Karner in New York State, where it was first discovered in the 1880s. Karner blue butterflies were federally listed as endangered in 1992 and remain a rare visual treat for entomologists.

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Psychedelic rock gecko -

No, not the name of some far-out 1960's pop group. Instead, the psychedelic rock gecko—only scientifically described in 2010 and a IUCN endangered species—is a bright yellow and orange reptile native to just two islands in Vietnam.

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Great raft spider -

A huge and hairy species of European arachnid, the great raft spider is semi-aquatic in its behavior. Found chilling out on unpolluted water of fens and grazing marsh, the spider is an increasingly rare sight, listed as it is by the IUCN as Vulnerable.

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Cuban crocodile -

This smallish reptile endemic to Cuba is known for its highly aggressive attitude and confident and rapid terrestrial locomotion. But the species is disappearing as fast as it can move: the IUCN lists the Cuban croc as Critically Endangered.

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Lord Howe Island stick insect -

Also listed by the IUCN as Critically Endangered, this species of stick insect, known alternatively as the tree lobster, is found exclusively on the Lord Howe Island Group in the Tasman Sea. It was declared extinct in 1920, only to be rediscovered in 2001. 

Sources: (National Park Foundation) (IUCN Red List) (Guinness World Records) (National Geographic)

See also: Are we facing an insect apocalypse?

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