|Use||Allows transformation to animal form|
The Animal Key was discovered by Bode Locke in a frozen fountain though it was originally hidden in the Hunting Trophy's Room. He used it to turn into a sparrow, and Dodge used to to turn into a wolf. Bode eventually used it to regain his human form. 
The key opens a door on the outside of the house (east side?). When a person climbs through the door, they are transformed into an animal and can communicate with other animals of the same species. It is unclear whether there is a way to choose what animal species you become, or if it is somehow a reflection on the person.
"...Ulysses said he would fly all the way to Hell if he had to, to find Delacorte for me. Clint said he would probably only be required to fly to Georgia, but that the two places were much alike, except Georgia is a bit hotter. We have fought a thousand times, my brothers and I, but this morning I felt I could not love any living souls more. Ulysses stepped through the door and emerged on the other side, a golden eagle. He gave me a short, lordly look, and took to the skies..."