"You have horses."
Translation:Du hast Pferde.
To clarify, if I said, "Du habt Pferde", this is wrong because I mixed formal and informal usages?
It's wrong because the pronoun and verb form don't match (singular informal pronoun with plural informal noun). It sounds wrong in the same way that "you has horses" does, as an analogy.
singular informal: du hast
plural informal: ihr habt
formal (either singular or plural): Sie haben