From the Zoopedia as written by Hendrix and Monschatten:

“To unlock the unicorn in challenge oder campaign the zoo needs a 5-star rating.”

“Today, Unicorns are mostly illustrated as white horses with a single horn on the forehead. But this is not correct. The best idea of the unicorn’s appearence can be found on gobelins. They show the unicorn as a deer-like artiodactyl. It has a slender neck with a long mane and a slim head. The horn is high on the forehead above the eyes. It consists of ivory and is twisted like a spiral. The ears are larger than those of a horse. Under the chin the unicorn has a beard like a goat.”