I've already written part of this once, but I had an unfortunate browser crash....
I first came across invisible creatures on the rmt-a newsgroups. They had a creature named the Invisible Purple Unicorn. More recently, some of my friends and I have begun seeing Invisible <Insert Color> Punchbuggies (a short aside: is it wrong to hit a woman when it's for a punchbuggie?). Anyway, today I came across a reference to purple cows over at Neil Gaiman's Blog for his upcoming novel, American Gods (from the blog of April 16th, 2001).
Purple Cow is a poem by Gelett Burgess. It reads:
The Purple Cow's Projected Feast: Reflections on a Mythic Beast, Who's Quite Remarkable, at Least. 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.
Apparently, Burgess became agitated when he found that he was known only for that work and wrote:
CONFESSION: and a Portrait, Too, Upon a Background that I Rue! Ah, Yes! I Wrote thePurple Cow-- I'm Sorry, now, I Wrote it! But I can Tell you Anyhow, I'll Kill you if you Quote it!
Parodies of the poem are available.

