1. Indian rhinoceros (Rhinoceros unicornis)
Also known as the greater one-horned rhinoceros,this is the largest of the three Asian rhino species, reaching 4m (13ft) long. As the name suggests, it also has the most impressive horn, rivalled only by the white rhino of Africa.
The Indian rhinoceros grows just one of these spectacular appendages. It is made of keratin, the same fibrous protein that makes fingernails and hair. Its horn can grow up to 1m (3ft 3in) long. This has made it a prime target for poachers throughout history.
According to the International Rhino Foundation, there is a common misconception that the powdered horn is used in Chinese medicine to treat erectile dysfunction. It is actually the outdated practice of using it to treat fevers and convulsions that makes it highly sought after on the black market. Intense hunting pressure pushed the species to the brink of extinction in the 20th century, with fewer than 200 left in the wild.
Now, thanks to strict protection, numbers have recovered to more than 3000 at the last count. But with 70% of the population in a single national park, this armoured unicorn is not out of the woods.
2. Saola (Pseudoryx nghetinhensis)
If you believe unicorns should be enigmatic, the saola fits the bill. It was only discovered in 1992, near Vietnam's border with Laos, when scientists found unique skulls in the homes of local hunters.
Analysis of the bones revealed the saola is related to cattle. Its name in Vietnamese means "spindle horn" in reference to the pair of parallel, straight horns, which can be 50cm (20in) long. Its nickname the "Asian unicorn" actually refers to its rarity.
The species has never been seen in person by a scientist. As of May 2015, it has only been recorded in the wild four times, using camera traps.
As a result their population can only be estimated, using remotely-captured photos, genetic analysis of dung and interviews with those living in the remote Annamite mountains. Experts estimate there could be fewer than 100 left in existence, their numbers under threat from hunters and fragmentation of the forest by roads.
They are listed as critically endangered by the International Union for Conservation of Nature, and conservationists are now striving to save them by appointing forest guards to crack down on illegal hunting.
3. Narwhal (Monodon monoceros)
In tales of the unicorn, much is made of the horn's magical properties. In nature, the narwhal's is arguably the most magical, as it has remarkable sensory powers.
The narwhal's tusk can grow to 2.6m (9ft) long, half as long as its body. It is actually not a horn, but an overgrown canine tooth that spirals counter-clockwise out of the left side of the animal's mouth.
No one is sure what it is for. It could be used as an ice pick, helping narwhals travel through the icy Arctic waters, or an acoustic probe or even a lance to battle for mates. Tusks are more common among males, so many think it is a sexual characteristic, with the largest tusk indicating the healthiest male.
In 2014, scientists analysed this tooth in detail and discovered it has no enamel. Instead, water flows through the tooth to nerve endings so the narwhal can sense differences in how warm and salty the sea is.
4. Unicornfish (Naso sp.)
Unicorns can also be found in warm waters, in the shape of the colourful tropical tangs known commonly as unicornfish. Half of the fish in the genus Naso, from the Latin "nasus"for "nose", have unicorn-like knobs on their heads.
This varies from a fleshy bump in the bignose unicornfish to an extended bony horn in the whitemargin unicornfish. The horn develops with age, forcing the fish to change their diets. Young unicornfish feed on algae on the seabed, but adults of certain species switch to snacking on free-floating zooplankton or fish poo once their horn starts getting in the way.
One of the largest species is the bluespine unicornfish, which can reach 60cm (2 ft) in length with a 6cm (2.5in) horn. It is a pale olive colour with two bright blue plates bearing curved, knife-like spines where its tail meets its body. These scalpel-sharp spines are for defense.
However scientists have so far failed to explain unicornfish's oversized noses.
5. Texas unicorn mantis (Phyllovates chlorophaea)
Several species share the name "unicorn mantis". It refers to a horn-like projection between their antennae. The horn is actually composed of two parts that grow side by side but do not fuse.
Among the best known is the Texas unicorn mantis, which lives near the US border with Mexico and grows to 7.5cm (3in) long.
It belongs to the Mantidae family, which are called praying mantises because they fold their forelimbs as if in prayer. "Preying mantis" might be more appropriate, though, as they are renowned for their big appetites.
The Texas unicorn mantis is an ambush hunter. Its body is a mottled grey-brown for camouflage, and its wings mimic bright green leaves to help it blend into the vegetation. It waits, completely motionless, using its impressive compound eyes to look for a meal.
When an unwary moth or grasshopper passes by, the mantis grasps it in a deathly embrace using its forelegs, which close together like the blade of a Swiss army knife. The mantis then slices its prey into bite-size pieces with its mandibles. Not exactly a fairy-tale ending.
6. Okapi (Okapia johnstoni)
In heraldry, unicorns have a patchwork of features from other animals: the body of a white horse, a goat's cloven feet and beard, a lion's tail and a slender horn. In nature, one animal rivals this.
It is horse-sized with a chocolate brown body, zebra-striped legs, large cow-like ears, a 15cm (6in) pair of horns on the males, and a relatively long neck. No wonder early explorers of the Congo region called the okapi the "African unicorn".
The neck is the clue to the okapi's true heritage: it is the closest relative of the giraffe. While giraffes live on the plains where leafy meals are served at height, okapis live in the rainforest where food is easier to come by. As a result, they are shorter than their lofty cousins and stand on average 1.7m (5ft 6in) tall at the shoulder.
Their camouflage stripes help them to blend in to the dappled shade of the forest. They are so well hidden, they remained undiscovered until the turn of the 20th century, long after most of Africa's other big mammals.
Despite being a protected species since 1933, the okapi is still hunted for bushmeat and threatened by habitat loss, so its future is uncertain.
7. Goblin spiders (Unicorn sp.)
Spiders of the genus Unicorn are named for a pointy protrusion between the eyes and jaws of the males, known as the clypeal horn. You'll need a microscope to see it, though. These spiders are extremely small, reaching a maximum body length of just 3mm (0.1in).
They live in high, dry conditions 1000-4000m (3280-13,123ft) above sea level in Chile, Argentina and Bolivia. Perhaps because they are so tiny, Unicorn spiders have only been known to science since the 1990s and little is known about them.
Female U. catleyi have been found with the broken tips of male genitalia lodged in their reproductive tracts. This suggests males of the species sacrifice their own genitalia as a kind of plug, to block others from mating with the female.
Unicorn spiders suffer from a mythical personality disorder, as they belong to a family called Oonopidae, commonly known as goblin spiders. Over 1,500 different species of these tiny goblins have been described from around the world, some of which are said to shimmer like glitter.
8. Helmeted curassows (Pauxi sp.)
Purists will argue that any unicorn worthy of the name should be able to fly, but they might not expect a unicorn bird. Helmeted curassows areturkey-sized black birds that live in the dense forests of South America. Each bird has a pale blue ornamental horn or "casque" on its forehead, which can grow to 6 cm (2.5 in).
The northern helmeted curassow (shown above) lives in Colombia. Its southern counterpart, which is technically known as P. unicornis, was thought to live in two separate populations: one in central Bolivia and the other 1000km (621 miles) away in the Sira mountains of Peru.
However, in 2011 scientists found that the Peruvian birds had shorter horns and shorter songs, and bred at a different time of year. They are now thought to be a separate species, P. koepckeae.
The newly recognised species is now considered critically endangered, as is the original unicorn bird.
"[The unicorn bird] is one of the most threatened species in the world, because of hunting and clearance of its forest home at the foot of the Andes," says Ross MacLeod of the University of Glasgow in the UK. "But it is a species we can still save from extinction with a concerted conservation effort."
9. Unicorn shrimp (Plesionika narval)
The unicorn shrimp is named for the elongated horn-like "rostrum" that extends in front of its eyes. The rostrumis covered in closely-set teeth and projects like a serrated red spear between its two white antennae.
Its scientific name is actually a nod to the other unicorn of the sea, the narwhal. While unicorn whales live only in the Arctic, the unicorn shrimp is rather more cosmopolitan. It is found off Angola, up to the Mediterranean Sea and all the way across to French Polynesia.
The unicorn shrimp only lives in cold water, and can be found down to a depth of 910 m (2985 ft), where it feeds on the sea bed. It uses its long antennae, which can be triple its body length, to pick up chemical clues.
It is considered a delicacy around the south-east Aegean Sea, particularly in Rhodes. However, following a drastic stock reduction, a project has been launched to find out more about the shrimp and manage it sustainably.
10. Arabian oryx (Oryx leucoryx)
The Arabian oryx meets so many of the criteria for a unicorn, it has been described as the "prototype" of the creature. Ancient authorities Aristotle and Pliny the Elder both described it as a one-horned animal but they must have been describing it side-on: the Arabian oryx has two horns that can reach 75cm (29in) long.
The oryx is white with a dark tufted tail and, as an antelope, it has cloven hooves. It can also detect rainfall and move towards it, which to travellers in the deserts of the Middle East may have seemed magical.
Like so many of the unicorn's namesakes, the oryx has been subjected to terrible hunting pressures, and in the 1970s was declared extinct in the wild. But it has since been brought back.
Following a successful captive breeding programme, Arabian oryx have been reintroduced to several countries including Oman, Israel and Jordan. There are now an estimated 1000 animals living wild again, so for now at least the Arabian oryx is more than just a myth.