Fastest speed over a quarter mile ~44mph/70 kph, fastest absolute speed recorded ~55mph/89kph, both values are for an America Quarter Horse bred for top speed over short distances. Horses in general just can't maintain the fast gallop for more than 3 or 4 kilometers before they need to slow down or even rest, so over long(ish) distances, the cyclists will win. Most races are for less than 3km.
Professional bike riders can maintain an average of 40-50 kph for several hours (average leg of 140 to 240 km), with top sprint speeds at the end beyond 70 kph. And they repeat that daily for several weeks.