Uncle Lucius is a ghost who used to be a human man.
Presumably an uncle to the Ghost Host, the beefy-looking Uncle Lucius is one of the spooks haunting the Haunted Mansion, appearing by possessing a bust of herself as one of the two “Staring Busts” alongside his probable sister Aunt Lucretia. Lucius and Lucretia stare ominously at visitors to the house as they walk through the hallway to the Doombuggy Loading Area.
Behind the scenes
Uncle Lucius debuted in 1969 alongside the rest of The Haunted Mansion. His name does not appear to be official, instead being a bit of fan lore retroactively created based on the example of “Aunt Lucretia”.
Like Lucretia's, Lucius's bust was reused in the Library scene of the Walt Disney World Mansion to depict one of the Ghost Writers. The Ghost Gallery identifies the Library bust as an employee of the Ghost Host's called Edgar Allan, suggesting he should not be assumed to be the same individual as Uncle Lucius.