Old Apple ID password recovery

I just got a new iPad (my first). I wanted to download all of the apps stored in iCloud so I would not have to download them 1 by 1. Seems that I have to provide the password for each of the Apple ID email addresses I have used to purchase or get free content. There's an old one that I cannot remember, nor can I find a way to recover the password or reset it. Entering the Apple ID in question which has not been used for years results in a message that it is not recognized. If it's not recognized, why am I being askd for the password on my iPad setup? Help please