This page explains how to start and complete the The Sheep Rustlers side quest.
How to Start the Quest
Talk to Koyin in Hateno Village to start the quest. She is at the entrance to the sheep pen in the eastern part of the town, just south of Lake Sumac.
How to Complete the Quest
If you make this quest active in the adventure log, the quest destination will show you the spot on Hateno Beach where you have to go. Go there and defeat all the monsters. When they're gone, their treasure chest will unlock. Then you can go back and talk to Koyin to complete the quest.
Quest Reward
Koyin gives you 10 Fresh Milk.