Depending where he came from I'd guess he has some local squirrel dog breeds from the south, Catahoula Leopard dogs and Plott hounds have some national standing, most of the rest are more localized like the Treeing Tenessee brindles and mountain curs. They'd certainly explain the coloring and even aggression. The fishhook tail with a crook in it reminds me somewhat of a rottweiler's tail.