I say, and with God's success: According to the opinion of Imam Abu Hanifa, may God have mercy on him, zakat is not obligatory on male horses when they are alone, as they do not reproduce; and similarly for females when they are alone in one narration. Refer to: Al-Ikhtiyar 1: 141, Al-Fath 2: 139, Majma' Al-Anhar 1: 201, Al-Waqayah p. 212, and God knows best.