"bullfighter on horseback" (as opposed to a torero, who kills on foot), 1610s, from Spanish toreador, from torear "to participate in a bullfight," from toro "bull," from Latin taurus (see Taurus).
A toreador is, or rather was, a gentleman who killed bulls for his own amusement on horseback and with the spear. He was a sportsman, and his sport was as manly and respectable as pig-sticking. A professional fighter who performs in a ring and for money is a torero. ["Saturday Review," Jan. 22, 1887]