Ammut (Ammit, Ahemait, Ammemet) was an Egyptian demoness. She was known as the 'Eater of Hearts', 'The Devourer' and 'Great of Death' because she was a demoness of punishment.
She had the head of a crocodile, the body of a leopard and the backside of a hippopotamus - all fierce creatures to the Egyptians. All man-eaters. It's no wonder that she was depicted as one who consumed the unworthy dead!
Of Ammut, an Egyptian papyrus (No. 9901) states:
hat en emsuh; pehu-s em tebt her-ab-set em ma.
the fore-part of a crocodile; her hind-quarters are those of a hippopotamus; her middle part [is that] of a lion.
In the The Book of the Dead, Ammut sat at the judgement of the dead in The Hall of Double Ma'at (when the deceased's heart was weighed on the scales against Ma'at), ready to devour the souls of the unworthy - the final death for an Egyptian! It has even been suggested that she was also a protector of Osiris, because of her position at the Judgement.
She was also known as the 'Dweller in Amenta' or the 'Devourer of Amenta', the place where the sun sets. Amenta, as used by the Egyptians, was applied to the west bank of the Nile - Egyptian cemeteries and funerary places were all on the west. To the Egyptians, west was a direction linked to death. Amenta was also the name of the underworld - the place where Ra traveled during the night. Ammut, therefore, was not only a demoness of death, but a demoness of the underworld. In at least one papyrus, Ammut was depicted as crouching beside the lake of fire in the infernal regions of the underworld!
The The Book of the Deadis a selection of spells, designed to assist the deceased through the trials of the underworld. This also, of course, assists the dead to not get eaten by Ammut. The papyrus of Ani, in a speech made by the gods to Thoth, says:
The Osiris [the scribe Ani], whose word is true, is holy and righteous. He has not committed any sin, and he has done no evil against us. The devourer Ammut shall not be permitted to prevail over him.