Moondancer is a previous-generation unicorn.
Moondancer appears in Generation 1 and 3, however is mentioned in passing in Friendship is Magic, Part 1. In Generation 1, she is a unicorn with a white coat and a crimson mane. In Generation 3, she is an earth pony with a shiny blue coat and blond hair. They both have a Crescent Moon and Stars cutie mark. Due to Hasbro loosing rights on the character, it is unlikely that Moondancer will make a physical appearance in Friendship is Magic.
Generation 1 Picture
Moondancer is mentioned in passing in Friendship is Magic, Part 1.