It was a bit confusing at first, but I could see this being expanded in a bigger game.
A nitpick would be that it feels like bringing your pet to maximum happiness would take a lot of time. Maybe the coins should drop more often, or the food and water should be cheaper.
Also I wish I could fight the enemies in the dungeons with my pets, but maybe that’s me being too pokemon-minded :D
Even so, this is a nice submission!