Thriller Bark

It's a fair assumption that such races ( or something similar ) presumably already exist in the form of the formally established snake-necks, so I'm thinking King may be something even more rare - a FEATHERED serpent/reptilian man; think Quetzalcoatl the feathered serpent Aztec God, or the famous lil "winged" dino Archaeopteryx.

King basically embodies a Missing Link of sorts that bridges different categories of living things: dinosaur and birds, reptile/snake-necks and bird-people.

Alternative theory that still ties back to a major Evolution theme: King is simply the avian equivalent to the Fishmen - a Tengu essentially. And just as Fishmen are denoted by traits specific to certain breeds of fish, I'd imagine the same would hold true for these hypothetical Tengu; King is a Phoenix-type Tengu.