You can see her on the steps of the throne of Osiris here (the b/w image is easier to make out). She has the head of a crocodile, the hind part of a hippopotamus and the middle part of a lion. Her teats show she is female. She also wears a modius, like Egyptian royal women. These picture are taken from out 21st Dynasty coffin (you can find out more about it here). She is waiting for the judgement from Osiris, does she get a nice tasty heart, or not?
However, there seems to have been more to her than simply a blood-crazed glutton of hearts. Strangely, perhaps, one of the beds in the tomb of Tutankhamun is in the shape of Ammut and he is called 'Beloved of Ammut'. Such beds in tombs would have been associated with resurrection.
Additionally, she often appears on 21st Dynasty coffins in scenes of Osiris' triumph over death, a scene that could also be said to be associated with resurrection.
Here she is on our coffin in the mound scene, where Osiris sits atop, reborn. Can you see her, front paws on steps? She is in the Amduat, the otherworld.
It seems however, that for the Egyptians, the destruction of enemies was necessary for rebirth, so Ammut is not so bad after all.
In some ways (her hippo links) she seems similar to Taweret, normally thought of as a good girl, but sometimes not so good (there is another tale there).