The Sheep Rustlers
Koyin is available on the path to the Hateno Ancient Tech Lab.
Talk to Koyin to start the quest. Follow the objective marker to the east side of Hateno Bay and kill all the monsters.
Return for a reward of 10 bottles of Fresh Milk.
You told Koyin that you cleared out the monsters living on Hateno Beach. In return, she gave you a handsome reward.
You feel like you're getting along quite well with Koyin now! Maybe?