I never saw a Purple Cow,
I never hope to see one;
But I can tell you, anyhow,
I’d rather see than be one.

That’s “Purple Cow,” written in 1895 by Berkeley drafting instructor Gelett Burgess. It grew so popular that it began to haunt him; eventually he wrote “Confession: And a Portrait Too, Upon a Background That I Rue”:

Ah yes, I wrote the Purple Cow,
I’m sorry now I wrote it.
But I can tell you anyhow,
I’ll kill you if you quote it.