The first few times I played through my Ruby, it was always on Groudon, and being young isn't really any excuse for me, since I usually did know it was best to save it for other ones (well, usually I just wanted to save it for Rayquaza, whereas saving it for Latios would've been a much better idea lol), and in the last playthrough I'd forgotten where it was even located due to it being so many years since I'd played it, and since I was in such a rush to get through the Magma hideout, I missed it, and didn't even get it that time. Ended up catching all the legendaries I was going for in regualr Pokeballs anyway lol