Pinkie Pony
Contest is the framing, not the mechanic. Pinkie Pony has entered a dance competition, but nothing in the game judges what you put on her.
That leaves a large mix and match wardrobe and a pony to hang it on, which is closer to a sandbox than the title implies. Dressing an animal body is a different problem from dressing a doll: the outline barely changes whatever you choose, so colour has to do the work silhouette usually does. Pick one strong colour and repeat it in two places.
Starsue.net built it, it plays free in the browser on a phone as well as a desktop, and there is no save, so screenshot the finished pony.
More games like this
Developer
Pinkie Pony is made in house, and this is its home. It is not licensed from anyone else.
Frequently asked questions
Is the dance contest played or scored?
No. It is only the reason for the outfit. There is no judge, no points and no ending, so you can keep restyling as long as you like.
Does it work on a phone?
Yes, it runs in a mobile browser as well as on a laptop, free, with nothing to install.