In Star Stable Online there are a select few fantastical horses that you can acquire for a limited time that comes with a special ability.
That ability is to change into and out of their magical forms!
Previously these horses changed automatically depending on where you were in Jorvik. If you were out in the wilds, they would go into their magical form, and if you were near civilization they'd change back to a more standard-looking form.
But now you are able to change their form whenever you want to!
To do so, you need to open up your character sheet (C on your keyboard) and look at your horses' sheet. There you should be able to see a button on the top left. Click on it and your horse should change form!