The Horseback Hoodlums
Talk to Perosa at the Highland Stable to start this quest.
Just across the grass from the stables are five Bokoblins on horses (unless you have killed some already). Kill all five (your objective status will update to let you know you are done) then return to Perosa for an Endura Carrot.
You defeated all the gang members causing trouble in the grasslands!
Perosa was so happy with the news, she gave you an endura carrot as a reward for your hard work.
It's said that horses are quite fond of those...