Iight, so here's my theory for King's Race. Just stick with me on this for a minute. You might like it:
WARNING: SPOILERS IF YOU'RE NOT CAUGHT UP ON THE MANGA
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The prevailing theory is that he is a Tengu. If you don't know what that is, go look it up. I personally think that's a little farfetched and has nothing to do with what's already established within the lore of One Piece. In my opinion, I think King is just another race of Sky People. Allow me to explain:
In chapter 238, Gan Fall said there was another way to reach Sky Island known as the Summit of High West. Now we know absolutely nothing about this Summit but we do know that the Red Line is west of Skypeia more or less.
Now we know from the Marco lore drop that King's race lived on top of the Red Line which is, as of now, the only place high enough to allow someone to get onto a Sky Island without the Knock Up Stream. I'm thinking that this Summit of High West is where King is from.
Now disclaimer: The rest of this is HIGHLY speculative which means I'm taking liberties with the canon coming up with this theory. Marco mentioned that King's race can conjure fire; an ability we have not seen among other Sky Peoples. As we know the Sky Peoples consist of the Birkans (Enel), the Skypieans (Gan Fall) and the Shandia (Wyper). Well, the Skypieans were considered to be angels and they even live on Angel Island. So we've got this angel motif right?
Well I propose that King is another race of Sky People that fit the motif of Lucifer/Satan the Fallen Angel. Lucifer was cast out of Heaven by God and became Satan. King's race was cast off of the Red Line by the gods i.e. the Celestial Dragons. Based on the way King's dressed, he ended up on the Blue Sea and became a warden of Impel Down (hell), hence why he's a torture-loving pervert. Just like Satan tortures sinners, King tortured prisoners in Impel Down.
Now this last bit gets even MORE speculative but bear with me a little longer: I think that while he was at Impel Down, King somehow met Queen who was working with MADS (i.e. the science group with Vegapunk and Vinsmoke Judge). They were able to extract his Lineage Factor and Judge used it in his research to create his children.
Hear me out: Besides users of the Mera Mera Fruit, who is another character who seems to be able to generate fire for no reason? Sanji. My theory is Judge's genetic tampering did actually work on Sanji and he was given King's ability to generate fire. Now why do I think this? Well in chapter 416 after Sanji defeated Jabra with Diable Jambe, what did he say? "Cooking is a gift from the gods. Spices are a gift from the devil." The "devil" being King and "spices" being the fire generation ability Sanji inherited from him. I'm just spitballin here, but hey.....