Horse
A horse walks into a bar. The bartender asks "Can I get you anything?" The horse replies "I think not," a promptly disappears.
If you don't get it, it's important to first understand that the French philosopher Descartes famously said, "I think, therefore I am." So when the horse said "I think not," then he could no longer be.
I guess I could have explained all of that before I told the joke, but that would be putting Descartes before the horse.