The Horse is an available pet in Harvest Moon DS: Sunshine Islands.
After raising Mirabelle's heart level to 5,000 (around 2 hearts), she will visit you on the farm one morning. She will offer the player one of two different horses to keep on the farm. One horse is white and the other is brown. The color of the horse is purely cosmetic, but the player cannot change their mind later if they decide they want a different horse.
The horse will live in the Stable on the player's farm and eat one serving of pet food per day. The horse cannot be used as a mode of transportation, and the player is unable to ride it. Instead, the Horse is able to compete in the Horse Festival that takes place every Spring.
When the horse reaches 2 hearts worth of friendship, the player will receive a Sun Stone. The horse needs to be fed and interacted with every day in order to increase affection. The horse is also the only pet that can be brushed for extra affection. A higher affection rate also increases the player's chances of winning in the Horse Festival. The mini-game for practicing for the festival can be found by interacting with the red sleigh inside of the Stable.
The horse will never die or get sick and lives on the player's farm indefinitely, as it also cannot be sold.