Michael Spivak's Calculus, lovingly referred to as "Spivak," is a textbook used by a number of universities in their honors freshman calculus courses. This iconic (if visually bland) text is noted for its dry, idiosyncratic humor. 

One compelling Spivak quirk is the concept of the "Yellow Pig." Leafing through the index, one finds, nestled between such technical mathematical jargon as "Partial fraction decomposition" and "Planetary motion, Kepler's laws of," the mysterious entry: "Pig, yellow, v, 371."  
We decided to investigate just how many yellow pig references there are inside Spivak. First stop: page v. Here we see a dedicatory page, bare except for this: "Dedicated to the Memory of Y.P." Is this a reference to a childhood friend? An influential colleague? A dear relative? No. It can be none other than the mysterious Yellow Pig.  
Trawling over to page 371, we see: "In this case we will go the whole hog..." While this is only an indirect reference, it seems Spivak was quite eager to enhance his own "yellow pig" mythology by listing this one in the index. 
In all, our search yielded 2 references to this infamous "yellow pig." For confirmation, we asked several Spivak devotees, possessors of the most intimate familiarity with the text, whether any more pig references lay concealed in the pages. Their answer: nope.