In Star Stable Online, there are fantastical horses that you can acquire for a limited time that come with a special ability: they can change into and out of their magical forms, and you decide when they should be magical or more neutral!
To do so on the desktop, you need to open your character sheet (press C on your keyboard) and look at your horses’ sheet. There will be a button up to the top left with a horse head surrounded by red stars. Click on it, and your horse will change its form.
If you don’t want to open the horses’ sheet, you can also simply press H on your keyboard.
Keep in mind that there is a 5-second delay between button presses, so you can’t spam the transformation. Also, if you log out and log back in again, the horse will always be in its magical form.