I don't know if anyone has mentioned, but I searched and have not seen much. But Bran could be based off the legend of Fionn mac Cumhaill a celtic legend picked from the wikipedia: Fionn's hounds, Bran and Sceolan, who were once human themselves I do not know if George is Celtic at all or has any Celt heritage but the dog having been human once and then Bran in the series being a warg with Summer would make this a good reference.