The actor playing Brock, the hay dealer that Frank owes money to, is in fact Frank's son-in-law, who is married to his daughter Christa and is the father of Beckett.
According to Seth Zeigler at Nokota Horse Conservacy, they have several traits in common with the Mustang horse breed because of their ancestry to Spanish colonial horses. Their robust physique sets them apart and they are well-known for their hardiness, good temperament and agility.