Since SCP-173 is the original SCP, I really want to ask: who wrote it? Was the poster anonymous, or is he here in the site right now, still active?
It pre-dates the site. It's a copy of a post on /x/ from about four years back. The author apparently registered a wikidot account a while ago and mentioned approving of what's resulted. I don't think he's been active since.
A poster named Moto42 claimed to be the original author back in an old thread, and elsewhere on the internet, but his claims are unverified.
All that is known for certain is that it was posted anonymously on 4chan. Anything else will be very difficult to actually prove. Which isn't to say I disbelieve Moto42 - but it'd take a lot of evidence to completely convince me.