Aunt Lucretia is a ghost who used to be a human woman.
Once the only female member of of the Singing Busts, the stern-looking Aunt Lucretia (presumably an aunt to the Ghost Host) is one of the spooks haunting the Haunted Mansion, appearing by possessing a bust of herself like the others. She does not appear to have been part of the chorus in forever, her bust instead appearing inside the house itself alongside that of her probable sibling fellow Staring Bust, Uncle Lucius. Another, seemingly-not-possessed bust of Lucretia also exists on the mantelpiece in the Ballroom.
Behind the scenes
Aunt Lucretia debuted in 1969 alongside the rest of The Haunted Mansion, as a Staring Bust — for all that she was originally conceived of as a Singing Bust by Marc Davis. In addition to her appearances as a Staring Bust in the corridor and as a non-spooky bust in the Ballroom, more haunted busts identical to her appear in the Library in the Walt Disney World Mansion, depicting “ghost-writers” alongside others statues. It is unclear if they were intended to also depict Lucretia in-universe; the Ghost Gallery establishes them to depict one Elizabeth Barrett, instead.