Kulthum bin al-Aqmar (may Allah have mercy on him) says: The first person to distinguish the Arab horses (in the shares) was a man from among us called Mundhir al-Wadi'i. He was the governor of Sayyiduna Umar (may Allah be pleased with him) over some areas of Syria. He pursued the enemy; the (Arab) horses caught up, and the non-Arab horses fell behind. So he allotted a share for the (Arab) horses and left out the non-Arab horses, and wrote to Sayyiduna Umar (may Allah be pleased with him), who replied: What you decided is excellent. So it became the practice. Imam al-Shafi'i (may Allah have mercy on him) says: Our view on this is that Arab and non-Arab horses, and the horse whose dam is Arab and sire non-Arab or the reverse, are all to be treated equally; and if we were to accept such a report as established, we would not oppose it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 17960