Equestria Online is a 3D My Little Pony Friendship is Magic MMO in progress. The current releases are being developed in Unity, though the team has not decided whether to use Unity 3D or Panda 3D for the game.
You can find the Game's website here: http://www.equestriaonline.com/
Or the Ponychan thread: http://www.ponychan.net/chan/collab/res/13261.html
The game is still in it's early stages, but still has plenty of progress to show off! The creators recently released a demo for Unity, which you can download here, which includes the player model, moving, the partially created map, and a few static NPCs.
A short demonstration of the game so far
The world of Equestria is literally being re-created for this game, and is planned to be as accurate as the show. There was a larger scale map created of Equestria that they will be using as reference, and screenshots and other maps for the smaller scale towns and cities.
Sweet Apple Acres
It has been confirmed that you will be able to not only customize your pony's color, mane, and cutie mark, but give your pony accessories as well, such as hats. Different outfits have been planned, and a few screenshots showing these off have been released, which you can find right here.
My hair is a castle, your argument is invalid
The team for the game consists of many different artists, music designers, programmers, and more. Each field has it's own separate team leader, ensuring that nopony will be overworked with multiple tasks for different things, and improving the overall quality of the project. You can find the complete team list here.
Unless changed in the future, the NPCs are planned to have actual voices, creating a need for voice actors. The voice auditions are currently closed, but you can find the original video announcement right here.
The music developed by the Audio Department are all completely original pieces, and can so far be found all on the Equestria Online's official Youtube page.
It sounds amazing, doesn't it?
The game does not have a planned release date, but they are making progress extremely quickly, so looking forward to seeing the released MMO in the near future isn't a far off dream.