The unicorn is a mysterious, majestic magical beast that resembles an imposing and beautiful white horse, with glowing eyes, a silky mane and tail, and the stereotypical spiraling, shell-shaped horn coming from its forehead. Unlike horses, they have cloven hooves, and males have a goat’s beard. The Dungeons & Dragons animated TV series features a baby unicorn named Uni in it.

The unicorn has been described since antiquity as a beast with a single large, pointed, spiraling horn projecting from its forehead. It has been depicted in ancient seals of the Indus Valley Civilization and even the Bible (the bible name it re’em, though). In European folklore, mostly during the Middle Ages and Renaissance, it was commonly described as an extremely wild woodland creature, a symbol of purity and grace, which could only be captured by a virgin. In the encyclopedias its horn was said to have the power to render poisoned water potable and to heal sickness. The unicorn was introduced in the «white box» set of 1974 for the very first edition of the game.

Society

Unicorns exist to protect forests from intelligent creatures. They dwell deep within their forests, and shun most creatures, only conferring with feys such as pixies, dryads, and sprites. They will show themselves to save their forests, however. Unicorns are strongly magical creatures, with most of their abilities coming from their horn. They can live up to thousands of years, and show effects of aging only weeks before death. They are able to sense evil nearby, and radiate an aura which protects them from most evil spells. The touch of their horn can both heal wounds and cure poison. Also, once a day, a unicorn can teleport to anywhere in its forest home (although they cannot teleport in from outside the forest). Unicorns attack by impaling foes on their horns, or with their hooves.
They are highly prized for the magical properties of their horns and their value as mounts. There has been no official way as to how D&D unicorns are captured, but in myth, they were lured from their homes with virginal maidens. Unlike most animalistic creatures, unicorns are capable of speech.

The Other Unicorns

In Faerun you can find Black Unicorns, evil counterparts created through magical breeding programs by the Zhents. Just like normal Unicorns, they will only allow human or elf maids to ride them, though their Evil alignment means that their rider must also be Evil (drow females are preferred riders).

In Eberron

House Orien is a human dragonmarked house throughout Khorvaire, with its major enclave located in Passage, Aundair. Its ceremonial crest features a unicorn, probably referring to its teleporting powers. The house specialises in courier services and land based transport. Though House Cannith invented the lightning rail, House Orien are the ones who operate and manage it.

House Orien has a large stake in the transportation business and a dominance over the postal business. It seperates these two businesses into two guilds, the couriers guild and the transportation guild. The couriers guild hires independant messengers and couriers to deliver letters and small parcels. Whilst the transportation guild prepares goods for shipment, running the lightning rail and maintaining the trade roads. The transportation guild is the largest dragonmarked guild and has tens of thousands of employees. It is rare for one to hire a Orien caravan to travel to a location of their choice as the price is often too steep. It is far more common to find travellers paying a fee in order to accompany a caravan along its pre-set route throughout a city or to a nearby settlement though, Orien often waves the fare for adventurers so long as they offer their services to protect the caravan. The house also runs a teleportation business though, due to its high fees it has yet to become a mainstream form of transportation.

In your campaign

Unicorns are fierce, intelligent creatures of the forest, noble beasts who keep their own counsel and typically appear only to defend their homes against evil. They universally shun all creatures except for good-aligned fey, good & pure-hearted humanoids, and the woodlands’ native animals, though they may fight alongside other good creatures against common enemies. Use this to have an aura of mystery and unreachability around them.

Unicorns mate for life, and the pairs generally make their homes in specific glades or dells within the vast forests they protect (these regions can cover anywhere from a few dozen square miles to hundreds). They allow good and neutral creatures to pass through, hunt for food, or reside in their woods unharmed, but evil creatures and those who damage the local ecosystem more than necessary through sport hunting or commercial logging are swiftly driven out or killed. On rare occasions, lone unicorns without mates or whose partners have been slain have been known to adopt young humanoids of exceptionally pure virtue as surrogates, allowing these youths to ride on their backs and becoming their guardians and protectors for life. This bond generally ends amiably if the person becomes more committed to someone else (such as a lover or child) giving rise to the myth that unicorns only befriend virgin girls.

You can take a look at the unicorn’s stats for 5e here.