Craven is a sorrel palfrey mare.
A Storm of Swords
After Sandor Clegane and Arya Stark flee the Red Wedding, they find the saddled horse a few days later. It is most likely that the horse's original owner was killed during the Red Wedding, and the horse fled. Arya names her Craven, due to the mare's meek nature compared to Sandor's fierce warhorse Stranger.
After Arya leaves the badly wounded Sandor Clegane, she rides Craven to Saltpans, where she tries to sell the horse. The woman who buys the mare recognizes her as a well-bred steed, and threatens Arya that if she doesn't accept the price offered, she will be reported to the castle for stealing some "good knight's horse". Since there were too many witnesses around, Arya had to accept the offer and the very flat purse. Arya feels the woman would not have dared cheat the Hound.