Buy food, water, toys, medicine, and shelter expansions.
Take dogs to the dog park for walks. Walking a dog until their "Fun" meter moves from red (negative) to green (positive) is the most consistent way to keep them happy. Acquiring the Game Animal Shelter Free Download (v1.2.7 & DLC)
Ensure you have the appropriate small-scale equipment (bowls, beds) purchased from the shop to accommodate these smaller residents. Essential Gameplay Tips Buy food, water, toys, medicine, and shelter expansions