Who is Seedy Sam?

Seedy Sam, at first seems to stand with the regular villains of the story; he clearly overworks and whips his horses. But Sewell affords him an opportunity to defend himself, and for one of the only times in the book, a character provides some sort of moral justification for treating these animals cruelly. Sam must choose between treating his family well and treating his horse well; he does not have the luxury to be lenient with his animals.