Maybe I'm the last person to know this but I just discovered that Myth has been ported to run native on OSX.
This free updater comes free from Myth Dev and they have released one for each of the 3 Myth games. These are actual upgrades to the original games. All you need to have is a copy of the onginal game on a CD. Once you do the upgrade you can also get a free account on Play Myth for network multi-player games.
OK, maybe I'll see you out there soon defending earth from The Fallen Lords and their minons!