At the top of the plaque is the evil face of an animalistic devil. He has wild hair, animalistic (goat-like) ears and small horns protruding from his forehead. He seems to be caclking in evil glee, allowing us to see his pointed teeth. The Devil wears a high Dracula collar. The general shape of this Devil's face is meant to resemble Vincent Price's.
The Vincent Price Devil heavily resembles the Devil on the plaque of the original Haunted Mansions in Disneyland and Walt Disney World. Unlike the HM Devil, the Phantom Manor Devil is however gleefully laughing. The HM Devil has bigger horns but human ears, and a less realistic face. He also does not wear the Vincent Price Devil's Dracula collar. Whereas the HM Devil is being strangled by two snakes, the Vincent Price Devil emerges victoriously from a mass of tangled snake who are actualy the framing of the plaque itself.
It is possible that the Vincent Price Devil is the Phantom's real form or one of the forms he likes to take; this is supported by the Devil's resembling Vincent Price, the fact that they both wear the same Dracula collar, and that the HM Devil is implied in concept art to be a representation of the Ghost Host, of whom the Phantom is a counterpart.