I logged into one of my side account probably a month ago. I've had this account for two years now, and I went to log in and it said the password was wrong, so I tried to change it and it said this account doesn't exsist. Last I tried making a new account with the exact same name and it let me, the name was gone so I was allowed to use it again. Can someone explain this?

Your account was deleted.

 

 

 

If you want a bit longer an answer, Jagex does regularly delete crappier, inactive accounts. I don't know the exact idling time required, though. But they do.