This page explains how to start and complete the The Giant of Ralis Pond side quest.
How to Start the Quest
First, you need to complete the Divine Beast Vah Ruta Dungeon. Then you can talk to Torfeau, who is using a spear in the area beneath the throne room in Zora's Domain.
How to Complete the Quest
Activate the quest in the adventure log to see the location of the Hinox. Go there and defeat the Hinox, then talk to Torfeau to complete the quest.
Quest Reward
Torfeau gives you a Silver Rupee.