At BroNYCon last Saturday they promised to release a beta of the single player MLP:Online Episode 1 on Monday. If we really want to be technical they did fulfil their promise by releasing just before midnight, although for the most of us that meant that we couldn't play until today. Is there a problem with that? Not really, and this way we should end up with a slightly less buggy beta, although from what I've heard you should prepare for your fair share of bizarre glitches. Remember to report any bugs to the team on the help and support forums, and that if a bug is humorous enough (or just really darn weird) then you should take a screenshot or record a video so that future generations can enjoy them too.
Right now the beta only supports Windows and lacks much of the content from the final game, such as the boss. I haven't tested the game in Wine yet but I think it should run decently enough. And as for the download, well you can find it at http://mlponline.net/download.php. Happy testing!