On this page of the Assassin's Creed Valhalla guide, you will find a detailed walkthrough description of the side quest A Dog's Rescue.
Rewards: 1,200 XP.
You will notice a burning hut at the location marked in the image above. There will be a woman kneeling in front of the entrance asking you to rescue her dog named Dandelion Puff, who was left in the burning building.
Go inside through one of the holes in the wall, then push back the heavy bookcase to open up an escape route for yourself (and the animal).
You will find the woman's pet behind a wooden wall. As it turns out, it is a fox. Walk your pet to its owner to complete this task.