Toffa at Outskirt Stable shares a rumor that skeletal horses can be seen in the Sanidin Park Ruins at night, and wants to see one.
Quest Location
You can start this quest by talking to Toffa at Outskirt Stable, -1449, -1251, 0032.
How to Complete the Quest
Sleep in a bed or sit by a fire until night. If it is already night, make sure that it is not too close to 4AM, or you might not have time to ride the Stalhorse back to the stable before it disappears at sunrise. From the stable, go north on the road and cross Manhala Bridge, then turn right at the fork in the road. You will eventually reach the park that Toffa mentioned. Sneak up on a Stalhorse and ride it back to the stable, and talk to Toffa while riding it. He gives you 50 rupees and a Pony Point.
Toffa then asks you to show him a tamed forest animal other than a horse. For example, there is a small forest on the hill northwest of the stable where you might find some deer. Equip any stealth-enhancing gear that you have, such as the Sheikah gear that you can purchase in Kakariko Village, and use stealth-enhancing food and elixirs if you have it. Then, carefully approach the animal from behind (but not directly behind, or it might kick you). When you are close enough, press the indicated button to mount, then tap the soothe button until the animal calms down. Then ride it to the stable and approach Toffa. You will receive 100 rupees and one Pony Point. This completes the quest.