The Unicorn Costume is a piece of decorative attire that was made to resemble the mythical being.
The costume has a foul odor radiating from it, according to Waffle. It is divided between the neck and the torso, for the consumer to enter in.
Unfortunately, a grizzly bear had spotted the "unicorn" and chased it down. It kept swiping and biting at the costume until it became worn out and torn up.
Once the bear was out of the scene, the costume was most likely left behind after the cats and Kimberly left the forest.