1) Very much like the mystical creature known as a Unicorn, but having a much larger ass. Either alongside it, as a friendly, blabbering and highly irritating companion or attached to it's rear as it's bum.
2) A rare crossbred mix of redneck cracker and stinky alien species.
Ned: "Hey Roger, did you just see that frikin Arsicorn?"
Roger: "What? What the hell are you talking about??"
Rilke: "Yeah, i thought i saw him buying a milk shake and gun earlier too."
Ned: "It has the hugest ass i've ever seen on a unicorn.."
Rilke: "Yeah, and a mullet!"
Roger: "Oh. There.."