The Treasure-Hunter is or was a human man.
This man was one of many treasure-hunters who came to the version of the Haunted Mansion existing outside New Orleans over the years, with the intent of seeking out the pirate treasure rumored to be hidden somewhere. He was stopped in the Stretching Room by the haunted hanging skeleton of the Ghost Host himself, who decided to tell him the story of the Mansion, the treasure, and his humble self — wherein which he had been the pirate Captain Blood. After finishing his story, the Host spooked the Treasure-Hunter with his usual door-disappearing trick, challenging him to “find a way out” and threatening him with becoming Happy Haunt #1000.
Behind the scenes
The Treasure-Hunter first appeared on the last page of the 2007 story Mystery of the Manse: Final Chapter, and is only ever seen from the back or in an extreme close-up (as pictured in the infobox). He is supposedly the individual to whom the Ghost Host is speaking throughout the Mystery of the Manse series, as essentially an audience avatar figure.