Of course it's not in the guide book. It's always some kind of glitch you have to exploit, or have to use action replay or gameshark. Wasn't in the guide book for red and blue, but what do you know, I caught one halfway through the game. Guide books aren't reliable when we're talking about something you have to use glitches to acquire.
way to revive an old dead thread. anyway, i just googled "how to get ho oh in emerald" and found this. http://www.psypokes.com/emerald/legendaries.php the one youre looking for is at the bottom.