Orc may have an actual name, but we don't know anyone who speaks Orcish. It all sounds like gibberish and grunts to us, so we just call him... Orc.
He seems happy enough with that arrangement.
Don't let the vacant stare fool you, though. There isn't a deep-thinking mind hidden beneath that startled gaze of his. This is surprising, considering the rest of the group turns to Orc for guidance and leadership. Not that Orc realizes this, of course. He's just happy to be hanging out with the object of his heart's desire - Monster Girl.