Horse

A horse walks into a bar and orders a drink. The bartender says, “Hey - I see you in here a lot. Are you an alcoholic or something?” The horse says, “I don’t think I am,” and then, poof - he vanishes like he never existed. 

Now this is a play on words about René Descartes’ famous philosophy of “I think, therefore I am.” But explaining that beforehand would be putting Descartes before the horse. #lamejoke