Unicorn

The unicorn is a legendary animal, commonly portrayed as a white horse with a spiral horn. It is a symbol of purity which could only be captured by a virgin, in this case is Yvaine. It was said to have the power to purify poisoned water potable and heal sickness, and repel dark magic, like the spells of a witch.

Appearence in the film
It first appeared when Yvaine was bound with a magic chain at a tree. The unicorn broke the chain with its magic and offered Yvaine, who had a twisted ankle from falling out of the sky, a ride. Yvaine and the unicorn rode to the inn, which Lamia created to lure Yvaine to herself. While Yvaine were treated by Lamia and her "daughter", the unicorn met Tristan. As Tristan was about to drink the poisoned wine, which Lamia´s "daughter" brought him, the unicorn ran against him and threw the wine flagon off his hand., saving his life.

Later the unicorn fought Lamia and the landlord, which was originally a goat. When the unicorn touched the landlord, he turned back to his original form, and he flew off to the bar of the inn ,knocking him against the wall (presumably killing him). Then the unicorn fought and injured Lamia. The unicorn was surrounded by green fire thrown at it by Lamia's attempt to trace it and the Star, but the unicorn did not die in the movie. It must have gotten away, because when Septimus finds his brother Primus dead, you can see the dead goat and the Primus are all left behind where they died. You never see the unicorn's body anywhere. This means the unicorn was able to get away after its's fight with Lamia.