The Howlers are obviously special ops both in and out of the Institute. Extremely loyal, inventive, and slightly crazy, they don't directly resemble any special ops force that I know of.
Knowing Pierce Brown, his books are littered with allusions to historical events and characters based off of important figures. I assume the Howlers have their base in some historical special ops. My first thought was the Green Mountain Boys of the American Revolutionary war, but they weren't known for being crazy. I think that's a hallmark of the Howlers- along with their extremely tightly-knit group, they're also constantly trying insane things.
Anyone have any idea what historical special ops force they could possibly draw their roots from?