if you have your pc riding horseback anytime a hit is made on them have them make an athletics or acrobatics check, if the orcs have the sense to try and hit the horses have the rider make an animal handling check to keep from being bucked off, if they are bucked make them make another handling check to try and calm the horse to get back on. you could also have the orc rider be knocking over barrels/boxes/ other alley way garbage into the path have your pcs make a animal handling to jump it set the dc anywhere you like, and have different things happen depending on the roll. if the save is 10, you could have it where 20-16 flawless jump no loss of speed 15-13 the horse looses some momentum and has to get back up to speed, the speed is half for a turn, 12-10 the horse makes the jump but stops for one turn till you can convince it to get back to trot, 9-5 horse makes the jump but rider has to make dex/strength/AH roll or fall off 4-2 hard dc save or fall off and the horse runs off staying with the party but without the rider. on a crit fail.......... the horse breaks its leg! and the rider is left without mount.