Mr Davis was possibly a human man who is now a ghost, though it is possible that he may never actually have existed.
The Ghost Host once told a story of the construction of his Haunted Mansion wherein which he ordered its building in life (as Master Gracey), hiring a duo of renowned architects to do so: Mr Davis and Mr Coats. Mr Davis was a small and cheerful man, while Coats was as tall as he was dark and brooding.
Both of them, however, were ambitious, and separately decided to use some of the booby traps they had built into the Mansion to kill each other, thereby taking all the credit for this one-of-a-kind manorhouse. Coats tricked Davis into the Coats Automatic Hydraulic Coffinizer, only to realize Davis had designed the hallways and corridors in such a way that without Davis to lead him, Coats would never find his way out of the house. Trying beyond death itself to get out of the coffin, Davis subsequently became the Coffin Occupant, while Coats, still searching for a way out even as an invisible spectre, became the Floating Candelabra spook.
This account of the Coffin Occupant's identity is far from reliable, however, as around the same time, the Host also told a completely different story of how he/Master Gracey became the owner of the house, wherein which it long predated his ownership of it.
Behind the scenes
Mr Davis appears in the 2005 comic story Blueprint for Murder. He is named after, and a caricature of, Mansion Imagineer Marc Davis, while Coats is Claude Coats.