Thunder Magnet is a side quest in The Legend of Zelda: Breath of the Wild. It is a quest given to Link by Cima at the Lakeside Stable. Link will receive a Rubber Helm as a reward for completing the quest.
When Link first talks to Cima, she tells the Champion that every time it rains she becomes unnerved as lightning strikes the stable without fail. Link then takes up her request to figure out why lightning hits the stable so frequently whenever a storm brews. He then climbs on top of the stable, and sees the problem: on the horse's head, there is a Woodcutter's Axe that is lodged into it and the metallic head of the weapon is what draws lightning to the stable.
The hero then takes the axe out of the horse's head and talks to Cima about why the lightning strikes happen so frequently, and in turn, solving the trouble and getting the Rubber Helm as a reward.