Flipyap

Flipyap is a Grunt mentioned by another Grunt in a long comment that can only be heard 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, now he's dead..."

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

The quote can be heard here: -d7IiiyukTc

There is also a chance with the IWHBYD skull on thatyou can overhear grunts talking, saying, "Cover me, I'm going in" "I'm not going to cover you, it's your fault Flipyap got killed"