You know I'm very interested in this sort of stuff. The things you will find in Betas is incredible. The original Pokemon Red/Blue was filled with crazy stuff in the beta. Even after production, Pokemon Gold/Silver still had Beta maps, and one of my favorite songs from the Gen II games. I've actually posted it for download on this same forum.
Now I saw a thread posted a long time ago (Sadly, it's now closed), which mildly proved Jirachi was a Beta Pokemon for Gold/Silver. Now although this seems a bit far fetched, take this into consideration; Gastrodon was originally going to be in Gen III, but was moved to Sinnoh. The Jirachi could be obtained through a rediculous cheat of some sort, and it was rock/flying, perhaps to give the conception of a meteor. My guess would be that it would be common at night in mountainous areas. Although many people believe it is fake, crazier things have happened in the Pokemon World. Mew cheat, Pomeg glitch, and the aforementioned Gastrodon.
Anyways hoped that tidbit of info helped! ^_^