Ammit the Devourer is an Egyptian demon.
Ammit the Devourer's preferred form is that of a hybrid monster with the anthropomorphized head of a crocodile, the forelegs and chest of a lion or leopard, and the backside and hind legs of a hippopotamus. She is very powerful, as her bite can unravel souls, essentially destroying them. Ammit was once a servant of the Egyptian Gods, and remains fearsomely devoted to them even so many centuries after their disappearance, unlike the One-Eyed Black Cat. As she would only use her fearsome abilities on their order, she is mostly safe to be around, though she may still lash out at one who, in her eye, unforgivably blasphemies her dead masters.
Ammit is not featured in any Mansion-related media, making her a Transplant Character, as she is (rather obviously) based on the figure of Egyptian mythology.