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 is 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.

The quote can be heard here: -d7IiiyukTc