Furies are Demons that have gone mad with bloodlust. During the War of the Forbidding, they broke free of the Forbidding with the help of the Dagda Mor. They tried to destroy the Druid Allanon during his first encounter with the Dagda Mor.
When they first appear in The Elfstones of Shannara, they are described as grotesque creatures, with bodies like a sinuous mass of grey hair, limbs bent and vaguely human, and their fingers grown into claws. Their faces are the faces of women with twisted features, while their jaws are like those of monstrous cats.
In later books, the Furies' cat-like nature would be emphasized, with the descriptions of them hunting and yowling being quite reminiscent of standard descriptions of lions and tigers.
In The Shannara Chronicles, Furies are depicted as beasts with bat-like wings, with little to no resemblance to feline creatures.