This is nothing more than a quick post about an aspect of the game that has bewildered many players (including me, initially!) In Island of Happiness, you could ride your horse... although Sunshine Islands follows Island of Happiness in many respects, you NEVER can ride your Horse in Sunshine Islands. It is not a question of allowing the Horse to become an adult as in earlier Harvest Moon games. Nor is it a question of Heart levels. Even at maximum 10 Hearts, you cannot ride the Horse.
The Pets who live in your Stable can be entered in Animal Contests. You can practice the mini-game (that constitutes the method by which the actual Contest is won) for each Pet daily. The Horse pulls a little sled in the mini-game and that is ALL he/she does in Sunshine Islands. The Dog catches a flying disc or 'frisbee'. The Cat plays with a toy on a stick.
Note that your chances of winning the Animal Contests improve as the Pets' Heart Levels increase but the 'Mood' of the Pet always is random. Even at maximum 10 hearts, the Horse can be out of sorts when you enter the Contest and resist. The Dog can display the 'black cloud' at 10 Hearts and be restricted to the distance to which this Mood will restrict him/her. The Cat may be inattentive and slow to respond initially...
Always save your game BEFORE you go to Meadow Island to compete in any Animal Contest that involves a Pet. Although training speeds will improve, the mood of the Animal during the actual Contest could be poor. YOu can win an Animal Contest even at a low heart level if you are lucky. You can lose it if you are unlucky even at maximum Heart Level. The chances of loading a GOOD Mood or GOOD level of attention initially improve with increase in Heart Levels but you always can be given a bad result at random...