The Heater is a tool used to keep animals warm and happy during the winter. It can be purchased from Marnie for 2,000g. A heater is the reward for completing the Fodder Bundle on the Bulletin Board.
A heater has an effect on animal Mood (which is different than Friendship) during the Winter when an animal is inside a barn or coop past 6PM, and the animal has 150 mood or above. Only a single heater is required to have an effect, additional heaters provide no additional benefit.
If the animal is able to enter a building with a heater without being trapped outside, they will gain 10 mood every 10 in-game minutes past 6PM, up to the maximum value of 255, or until the Farmer goes to sleep.