Flipyap

Flipyap is a Grunt mentioned by another Grunt in a long comment presumabely unlockable only with the I Would Have Been Your Daddy Skull.

"You killed Flipyap! Or Yapflip, was he... It was Yapflip... No, Flipyap was his brother. Don't tell me I don't know Flipyap! Flipyap and I went to Nipple Academy together, and, eh, he's dead..."

Flipyap and the Nipple Academy are most likely only meant for humorous purposes by Bungie.

http://youtube.com/watch?v=-d7IiiyukTc