There first sighting (off minas sombra) was on the world of Seppius where they were used simply as tansportation but during the aureillans rise to power they were corrupted by evil and killed much of the planets poplace. The creatures stripped the planet of many of its resouces and then when there was nothing left began to eat one another.
Around the middle of the Galactic War,they were found by aureillans who were visting only to found the cities destroyed and ablazed and the dragons freely roaming. It is seen by the aureillans as a chance to expand there army so they simply took the creatures. for the rest of the war they were unleashed on particular worlds that the aureillans desired. When the war was over they were amongst the aureillans' most feared creatures.
Clearly, Fell beasts were powerful creatures and were deadly offensive predators, especially with the Nazgul on them. They could choose either to bite their prey (less attacking radius but increased damage) or swoop down on them (larger attacking radius), scattering enemies apart or outright crushing them. If a Fell beast snatched a soldier, they could easily fly high into the air and drop their hapless victim to his death far below.