What's a yellow pig? Well, what's a purple cow? A yellow pig shouldn't be too hard to imagine. Now, if you've seen a yellow pig, you are very lucky. Yellow pigs are rare because for some reason people are predisposed to think of pink pigs as pink or brown or even black but never yellow. There are some real yellow pigs, but they are rare. Not quite as rare are yellow pig statues, pictures, and stuffed animals.
Many people collect yellow pigs because they understand just how special these porcine creatures are. Yellow pig followers may choose to worship Her Majesty the Yellow Pig, but just as often they are more secular and instead work on math problems. They may celebrate Yellow Pigs Day by singing yellow pig songs. Most yellow pig followers have attended David C. Kelly's awesome Hampshire College Summer Studies in Mathematics (HCSSiM). If you have a friend with a strange predilection for yellow pigs, chances are s/he went to this summer program.
Yellow pigs have seventeen eyelashes. That's eight on one eye and nine on the other eye, but I refuse to say which because something must be kept a mystery. (That and they tend to change when the yellow pig flies through other dimensions.) Rumor, legend, and lore have it that the yellow pig was dreamed up by Michael Spivak "in a bar across the street". I suspect it was a lot like seeing pink elephants. There are references to yellow pigs in several of his books on differential geometry. But when I questioned Kelly about the conception and history of the yellow pig, he simply replied "If you have to ask, you just won't understand".