How to Pick Up Request 19: A Peculiar Ponyta
You’ll need to have caught a normal first. These can be found in the Obsidian Fieldlands, notably near Horseshoe Plains. Also, you may need to get Research Level to 10.
After learning the request, head for the bridge near the north-west of Jubilife Village, south from the Move Tutor. Speak to Yota and he’ll explain the details. Basically, he wants you to investigate a strange (spoiler: it’s shiny) at the Horseshoe Plains.
Where to Find the Shiny Ponyta
Without further ado, head for the center of Horseshoe Plains at the Obsidian Fieldlands. For an easy time, set this request as your active target from the map (by pressing the Y button and switching to the Requests tab). A quest marker will appear to guide you to the exact location.
Once you reach the destination, a short cutscene will play, where your character spots a shiny . When you’re in control, carefully approach the . There will be a body of water between you. What you can do is distract it with an Oran Berry etc., then sneak behind it and throw a Poké Ball.