Unicorns

Unicorns are one of four kinds of ponies on My Little Pony Friendship is Magic who live in Equestria. They are characterized by their horns and their ability to perform magic.

Development
Unicorns were introduced after Earth ponies in the original My Little Pony toy line. In previous animated series, they had the ability to teleport magically or "wink out", but did not ordinarily have any other magical abilities.

Unicorns originate from the writings of Helenistic Period of Greek history. They were considered to be part of India's native fauna. During the Middle Ages, unicorns were linked to several animals, among them the rhinoceros, the oryx and the mammoth. Medieval legends relate that unicorns permitted virgins to ride on their backs, while subsequently killing imposters. In modern times unicorns have become associated with magic and beauty. Monoceros (Latin: Unicorn) is a constellation in the celestial equator, and one of the 88 modern constellations.

Depiction in the series
As the name suggests, all unicorns have a long horn protruding from their forehead which glows when their magical powers are used. Although they are associated with their magic, a unicorn's horn is strong and sharp enough to have mundane uses: one unicorn in Call of the Cutie uses it to pop balloons, and Twilight Sparkle uses it to charge an opponent on several occasions. Twilight Sparkle and Rarity are often seen using their intellects to compensate for their lack of physical strength, such as when Twilight earns fifth place in the Running of the Leaves in Fall Weather Friends and Rarity overcomes her captors in A Dog and Pony Show. It was stated in Boast Busters most unicorn magic revolves only around their natural talents (e.g. their jobs, hobbies, etc.) as seen in Rarity's case, thus making unicorns like Twilight Sparkle unusual for having a talent that is magic itself. However, both Rarity (in The Cutie Mark Chronicles) and Twilight Sparkle (playing the role of Clover the Clever in Hearth's Warming Eve) have stated that Unicorn magic itself does not happen without reason.

Also, four times in the series, their magic has been cancelled out. The first is when Twilight Sparkle is affected by the Poison Joke plant in Bridle Gossip, wherein the plant makes Twilight's horn floppy, rendering it unable to do magic. The second time is in The Return of Harmony Part 1, when Discord removes the horns from Twilight Sparkle and Rarity. In Ponyville Confidential, Sweetie Belle simply kicks Rarity's horn to cancel out her levitation spell instantly, while in The Crystal Empire - Part 1, Shining Armor's magic is rendered ineffective by King Sombra's magic. Sombra's causes black crystals to grow on Shining Armor's horn, nullifying spell effects. The other pony races generally have difficulty competing with a unicorn's magical talent, as seen when Trixie easily bests Applejack and Rainbow Dash in a show of skill. This causes Rarity to state that only another unicorn could put Trixie in her place, although Twilight Sparkle is the only unicorn present able to outdo Trixie.

Alicorns
Faust intended Princess Celestia and Princess Luna to be the only Alicorns. They are called unicorns in the prologue in Friendship is Magic, part 1, and Luna is called a "Pegasus unicorn" on Hasbro's My Little Pony Facebook page. On merchandise, the princesses are called "Pony Princesses", including Princess Cadance; in A Canterlot Wedding - Part 1, Twilight Sparkle contrasts Cadance being a "princess" with herself being a "regular old unicorn"; the script for A Canterlot Wedding Part 2, when naming Queen Chrysalis for the first time, refers to her as a "pegacorn". Despite these other terms, storyboard artist Sabrina Alberghetti calls the princesses "alicorns". In Magic Duel, the word "Alicorn" is used for the first time in reference to an amulet depicting a Pegasus unicorn. Twilight Sparkle's transformation and subsequent reappearance to her friends in Magical Mystery Cure is the first time any character is directly referred to as an Alicorn in the show.

Unicorn magic

 * For a comprehensive description, see the unicorn magic section in the article magic.
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Residence
Canterlot is mostly populated by unicorns in its three appearances in Friendship is Magic, part 1, Sweet and Elite, and The Cutie Mark Chronicles. Notable exceptions are Hoity Toity and Princess Celestia's pegasus pony guards. Ponyville has its share of unicorns, more or less equally with Earth ponies and Pegasi. In Applejack's visit to Manehattan unicorns are only seen as waiters, and the rest of the ponies there are Earth ponies. Since they lack the natural ability to walk on clouds, they should not be ordinarily present in Cloudsdale, but Twilight Sparkle casts a spell on herself and her friends that allows them to walk on clouds and attend a competition in Cloudsdale in the episode Sonic Rainboom.