"My horses do not eat rice."
Translation:Mis caballos no comen arroz.
sus would be used if they were somebody elses horses, but in this case they are your horses. sus caballos no comen arroz would be his/her/your/its/their horses do not eat rice. su caballo no come arroz would be his/her/your/its/their horse doesn't eat rice. su/sus can refer to "your things" as well as"their things" but it does not refer to "my things"