Founder = equity on day 1 or first couple of days/weeks/months

Co-Founder = more than 1 of above

Owner = equity earned after (sweat or money), founders are automatically owners as long as you still have shares

You can be a founder but no longer own the company (sold all shares)

If you have to take money from your own pocket to keep the company going, you are an owner - capital call anybody?

Some people actually don't want to be called founders as they think that the title they give people when they have put out to pasture and no longer actively contributing :)

But titles don't really matter unless that is all you are after