Flipyap

Flipyap is a Grunt mentioned by another Grunt in a long comment presumably unlockable only with the IWHBYD 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, now he's dead.....catch!"

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

The quote can be heard by following this link: http://youtube.com/watch?v=-d7IiiyukTc