Unicorn

A unicorn was a mythical beast similar in appearance to a horse, but with a single long horn in the middle of its head.

The Second Doctor encountered a unicorn in the Land of Fiction. (DW: The Mind Robber)

The Ceffyl of Tír na n-Óg resembled unicorns by design. (NA: Cat's Cradle: Witch Mark)

In 2002, unicorns appeared in San Francisco due to a dimensional scar. (EDA: Unnatural History)

"The Unicorn" was the alias of Ada Mullins. (DW: The Unicorn and the Wasp)

The Bestiary of Legendary and Magical Creatures created a unicorn out of toilet paper. (ST: Phoenix)

Iris Wildthyme had a unicorn statuette in her TARDIS. (ST: Old Flames)

The Doctor once claimed he was a unicorn (NSA: Borrowed Time)

Behind the scenes

 * According to The Brilliant Book 2012, a book that contains non-narrative based information, in River Song's World unicorns were a pest in London, alongside Cyclops and Satyrs. (REF: The Brilliant Book 2012)