It is possible to get Ho-oh in Pokemon Colosseum. What you have to do is in the main menu, go to Colosseum Battle->Solo Battle. In this menu, register your pokemon (make sure to pick Colosseum Pokemon)(you can't if you have Shadow Pokemon in your party). Now, back to the Solo Battle menu, select Mt. Battle Vs. 100. Complete it (it is kinda hard) and Ho-oh will appear in your box in Story Mode. Note that this does not work if you registered your GBA Pokemon. If you do that, no one gets a Ho-oh, not even your GBA. This problem is easily solved by trading your GBA Pokemon to your Colosseum save (you have to beat Story Mode and save after the credits to trade with GBA games). Good luck with Mt. Battle! Oh, and you don't get this for beating the Mt. Battle in-game. You have to beat this Mt. Battle. The in-game Mt. Battle gets you a Time Flute.