I Say he's a Lunarian; Since If He DID eat the Gomu Gomu no Mi/Hito Hito no mi Model: Nika; that's Probably WHY Luffy can Use Fire; even though he doesn't have Wings like King, the Final Lunarian did; and Didn't eat The Mera Mera no Mi, like Ace & Sabo did; and He was Probably a HUGE one; If He really DID wear that Giant Straw Hat in the Freezer We See Imu in. . .

All this hype is only going to just turn Joyboy into a regular human in the One Piece world, kind of like Shanks..

I just assume that he was a human or part of a race that has yet to be revealed in the series.

Joy Boy is the ancestor of the D clan.

Some people claim that Joy Boy is either immortal, a time traveler, or a different being.

No Human can Emit Fire like a Lunarian(WITHOUT having eaten the Mera Mera no Mi)

No Human can Wear a Hat as Gigantic as the one in Imu's Freezer

I believe he was either a human or giant