Who is Sayf ibn Hubayrah, and was he one of the horsemen of the Messenger of Allah, may Allah bless him and grant him peace?

The name mentioned is not among the Companions (Sahabah). Perhaps the questioner means Amr ibn Hubayrah, the courageous Amir whose sword became proverbial. Al-Dhahabi translated his biography in "Siyar A'lam al-Nubala'" as Amr ibn Hubayrah ibn Mu'awiyah ibn Sakin, Abu al-Muthanna al-Fazari al-Shami, the Amir of the two Iraqs, and the father of Yazid. He was in charge of naval expeditions, then was dismissed, imprisoned, escaped, sought refuge with Maslamah ibn Abd al-Malik, and passed away around the year 107 AH.