On this page of the Assassin's Creed Valhalla guide, you will find a detailed walkthrough for The Wayward side quest.
Rewards: 1,200 XP.
Go to the place marked in the picture above and you will meet a girl named Asta. She claims to be the greatest summoner and calls her steed, Skinfaxi. Unfortunately, the animal doesn't listen to her.
Talk to the girl and then go to the other side of the river – you will find the white horse. The animal is marked in the picture above.
Bring the horse to the girl and then get off the animal to complete this task.