Anypony familiar with the movie Mannequin from 1987? The story of a young princess Emmy, wishes for a better life and time travels from ancient Egypt to the future where she takes the form of a mannequin built by a young wannabe artists Jonathan, and the two fall in love.

My idea is a similar story, Rarity gets a special mannequin to decorate for somepony and the mannequin comes to life and is a prince from long ago, and they fall in love and she is the only one who can see him alive but nopony else can and her friends get worried about her thinking she’s crazy.