Monster design based on the bird of the same name. This nighthawk, unlike its real life counterpart, doesn't do much flying. However he's still a stealthy nocturnal hunter. He just loves bursting out of the undergrowth upon unsuspecting prey.
The wings are used in place of forelimbs, and for fighting and hunting. The leathery feather-like planes that extend from them are leftover from a time when they very well may have flown. For now, they serve other purposes, such as for communication and mating displays. I can also imagine them serving some heat regulatory function, as well as the creatures spreading their wing leather out to extend their already long leaps.