Well that "other hole" is probably it!
Just because you haven't had sex or used a tampon doesn't mean that it is definately going to be small. The hymen can get stretched and torn just by general physical activities such as horseriding so the hole can just as easily be quite big! Everyone is different.
Your urethra is very small and barely visable so if you can see the hole and think it is big then it is not that. So the only other options that the hole is your anus or the vaginal opening. The anus is a pretty obvious hole because it is not surrounded by skin folds and it a lot further backwards. It is surrounded by smooth (or maybe a little hairy) skin. It would seem to me that the other hole you found is the one you are after!