The subject of this article originates from non-canonical sources. To find out about what is considered "canon" see LOTR:Canon.
The Vale of the Fellbeasts was a plain in northern Mordor. In this valley the Dark Lord Sauron nurtured certain creatures with unknown things, which grew and became the Ringwraiths' flying steeds after the Nine lost their horses in the Ford of Bruinen.
The name and location of this valley were not created by Tolkien.
- Vale of the Fellbeasts at Middle-earth Role Playing Wiki