In the nearby village of San Juan del Obispo [1], toward Agua Volcano, Ravenscroft Riding Stables (2a Avenida Sur #3, San Juan del Obispo, tel. 7832-6229) offers three-, four-, or five-hour rides in the hills and valleys near Antigua for $15 per person per hour.
Links:
[1] http://www.moon.com/destinations/guatemala/antigua/near-antigua-guatemala/san-juan-del-obispo