Captain Flint was a human man.
Captain Flint was a feared 18th century pirate, who, with his crew of cutthroats (quartermaster Long John Silver chief among them), accumulated an enormous fortune. Flint buried his loot on an island known only to himself, known ever after as “Treasure Island”. A cruel man with a twisted sense of humor, Flint killed the five men who'd helped him buried the treasure, leaving their skeletons on the island, positioned so as to point the way to the treasure. Flint then died, leaving the map to rival pirate Captain William Bones — a poisoned gift, as it set now-captain Silver and Flint's old crew on his tracks.
Behind the scenes
Captain Flint is heavily mentioned in the 1950 film Treasure Island and a few other is adaptations and parodies of the Treasure Island story, as well as its sequel Long John Silver. He is also mentioned in J.M. Barry's Peter Pan, which inspired the Disney animated film of the same name.
In Stevenson's original novel, Flint's name begins with "J"; his Disney (but non-canonical) appearance in Muppet Treasure Island gives him the name Bernie Flint, while his counterpart in Treasure Planet is known as Nathaniel Flint. In Treasure Island, Flint is portrayed by the Phantom Blot.