What is a zebra?

This is funny-- the cartoon is edgy, so be warned if you're easily offended [racial humor], but what is really funny is the comment from one reader, atavist, in response to a posted complaint. Also, you have to appreciate the author's attempt at pure humor outside of the touchy sociological context:

"Is a zebra a white horse with black stripes or a black horse with white stripes?

Answer: A zebra is a gray pony that is not vibrating fast enough to appear gray"