The Horseback Hoodlums
To unlock this sidequest, talk to Perosa at the Highland Stable. She will tell you that there are five Bokoblins riding on horses causing mayhem. It would be great if someone could take care of them and it looks like that somebody is you.
Just a short distance from the Highland Stable is a vast, open field cut through by a footpath. The Bokoblins are riding around there. You certainly don't have to kill the horses to get to them, but if you do accidentally hit a horse once or twice, it will simply run off with no lasting consequence. The real target is obviously the Bokoblins so dispose of them however you want to.
If you are having trouble keeping up with them, especially the ones using spears and other longer weapons, try using your bows to shoot them down from afar. Or, get a horse of your own and chase after them, then give them a good whack with your own melee weapon.
Once all five Bokoblins are dealt with, report back to Perosa for your reward, an Endura Carrot.