Craven is a sorrel palfrey.
A Storm of Swords
After Sandor Clegane and Arya Stark flee the Red Wedding they found the saddled horse a few days later. It is most likely the horses original owner was killed during the Red Wedding and the horse fled. Arya named him Craven due to the horses meek nature compared to Sandors fierce warhorse Stranger.
After Arya leaves the badly wounded Sandor Clegane behind she flee's on Craven to Saltpans where she tries to sell Craven. The woman who buys the horse recognises it is a well bred steed and threatens Arya that if she doesn't accept the price offered she will be reported to a Lord for stealing some "Knights horse". Since they were too many witnesses around Arya had to accept the offer and the very flat purse. Arya felt the woman would not have dared cheated the Hound.