Impa has been raising a mare to be your loyal steed. Impa will send word to Hyrule Ranch so you can pick up your new horse.
Where to Start the Quest
After you complete the quest "Still Missing", and the side-quest Runaway Horse, you can talk to Impa in the throne room of Hyrule Castle to begin this quest.
How to Complete the Quest
Go to Hyrule Ranch and talk to the woman in the northwest corner of the pasture who has a carrot accessory in her hair. There will be a cutscene.
Afterward, your new destination is northwest of Hyrule Castle. You will find a rift with a glowing spot in it. Check on the glowing spot and check on the entrance to go in.
Stilled Carrot Patch
Go north and pick up the northeastern box to reveal a ladder. Go down. Get past the Platboom. If you haven't learned the platboom echo, you can use a monster echo or swordfighter one to defeat the Platboom and learn the echo. Then create a Platboom in the vertical tunnel on the right side of the room. Jump on top of it, then bind to the large boulder on the left. When the Platboom pushes you up, the boulder will also move up. Go to the left with the boulder so it goes onto the button. Then use echoes to destroy the wooden boxes beneath you. Check on the glowing object to rescue one of Tri's friends. Destroy the boxes to the left and go up the ladder.
Go west and pick up the leftmost ladder on this island. Go down the steps. Pick up the meat in the southwest corner and throw it where both dark Moblins will see it. While they eat, you can defeat the Moblins. After you defeat them both, check on the glowing object to rescue another of Tri's friends. Go back up the stairs.
Go west, then north. There are four large boulders here. You need to pull the northern boulder to the south. To make this easier, you can create a trampoline echo, then bind to a boulder, then bounce on the trampoline to help get the boulder over the stones. After you move the northern boulder out of the way, check on the glowing object to rescue another of Tri's friends.
Go to the east side of this island and use a Platboom or other echoes to reach the islands above. Then go northeast on those islands and check on the glowing object to rescue the fourth of Tri's friends. There will be a cutscene.
Quest Reward
Tri's level gauge will increase, and you get 2 Might Crystals.
After the cutscene, bind to a carrot in the carrot patch, then pull on it to harvest it. There will be a cutscene. Before you go, pull up another carrot and learn the carrot echo. You can now create a carrot echo to summon your mare no matter where you are. If you use a carrot but your horse doesn't appear, check the map to see if your horse is nearby and wasn't able to find a path to you.