Creation

Goatboy was created by writer Grant Morrison and first illustrated by Chris Burnham.   
 
His name was inspired by the literal meaning of Gotham City, "Home of Goats" , so he took the pseudonym "Goatboy" from "that Bill Hicks sketch".
 

Origin

Following the half-a-billion dollar bounty that was placed on Damian Wayne's head by Talia Al Ghul, many assassins are on the hunt for the Boy Wonder. One of these people is Goatboy, an average taxi-driver, father and killer-for-hire born and bred in Gotham City. He claims to understand the workings and machinations of everyday business, so he knows where and when to be at the right time to make a hit. 
 
After using several doppelgängers to misdirect a pursuing Batman and Robin, he snipes at Damian from a distant rooftop with his long-range rocket rifle. However, he misses his target by mere inches.  
 

Later on he stakes out a gathering of Mutants sorting out a tainted beef delivery, awaiting the Dynamic Duo. Although he is jumped by Batman, he manages to shoot the Dark Knight in the face, and then in the moment shoots Damian in the skull. 
 
He captures his hit on camera phone and pleads Leviathan for protection from a furious Batman.