This page explains how to start and complete the Thunder Magnet side quest.
How to Start the Thunder Magnet Quest
Go to Lakeside Stable (near Shai Utoh Shrine) and talk to Cima, who is standing near the horses, to start the quest.
How to Complete the Thunder Magnet Quest
Climb onto the top of the stable. You can climb up the neck of the horse to get there. Or you can wait for an updraft if Farosh flies past. Or you can use Revali's Gale if you have it.
On top of the stable, on the horse's nose, there is a Woodcutter's Axe. Remove the axe from the roof of the stable, then go talk to Cima to complete the quest.
Thunder Magnet Quest Reward
After you remove the axe from the roof of Lakeside Stable, Cima will give you the Rubber Helm, which gives you Shock Resistance.