The Jayhawk is a weapon Cywren used in the Capital Wasteland.
The Jayhawk first appeared in the third episode of A Fallout Tale, when Cywren had just arrived in an old town mostly broken from the affects of the nuclear bombs. She found it on a Raider, along with the Elephant Gun.
Cywren does use other weapons, like Lincoln's Repeater, but she still says that the Jayhawk will be her favorite weapon.