The Bog Unicorn is a horse that came back from the dead with a sword stuck through its head. It is a mount that can be used by the Inquisitor. The Bog Unicorn is available as part of the DLC that comes with Dragon Age: Inquisition Deluxe Edition and Dragon Age: Inquisition Inquisitor's Edition.
Official description
Once belonging to an evil marauder, this unique mount has returned to inspire fear into those who would oppose you.
Acquisition
You must first initiate the quest Horses for the Inquisition and complete In the Saddle quest, before you can see the Bog Unicorn mission on the war table (Ferelden) to unlock this mount.