The Mad Scientist is the ghost of a human man.
The ghost of this monstrous-looking mad scientist, still manipulating phials of dangerous glowing liquids, is one of the residents of the Haunted Mansion.
Behind the scenes
The Mad Scientist was a possible character for the Haunted Mansion ride, sketched by Ken Anderson in the 1950's. Though not implemented, he inspired a later character, Phantom Manor’s apothecary Doc Jekyll. He may also have inspired the Ghost Host's ownership of a lab (complete with Igor) in Episode 2 of Season 1 of the unlicensed Haunted Mansion Show, Halloween Preparations.