وَلَيْسَ فِي ذُكُورِهَا مُنْفَرِدَةً زَكَاةٌ عِنْدَ أَبِي حَنِيفَةَ رَحِمَهُ اللَّهُ
Translation: According to Abu Hanifah, an all-male herd of horses is not subject to this zakat.
Munfaridah specifies males standing alone without mares. One legal explanation concerns the absence of breeding within that herd. This is not an exemption for horses held for resale.