User blog comment:WildBrick142/New GTA V Screeshots: The Fast Life/@comment-5733239-20130729210119/@comment-4095569-20130729222027

It appears to be their own varient of a  CH-47 Chinook mixed with a MH-53 Pavelow.

Possibly due to copyright reasons or due to wanting to add both helicopters in the game but only needing one implemented. Either way, it looks awesome.