Jamie Braddock manifested his mutant power of quantum manipulation in Excalibur (1st series) #15 after being tortured by Doc Croc for months. Believing himself in a dream world due to his trauma and the nature of his newfound powers, the callous Braddock operated with absolute disregard for the lives of others. He eventually recovered as a force for good in Otherworld, but was destined to become the Goat after exposure to malignant magicks, threatening all of reality. His sister Betsy was forced to kill the younger, present Jamie in Uncanny X-Force (1st series) #23 to erase the Goat future timeline. He was resurrected through the efforts of Krakoa in Excalibur (4th series) #1.
House of X #1 identified Jamie Braddock by the codename Monarch in their Omega Mutant Files, but he has yet to adopt that codename in the stories themselves.