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For starters, I tried to use the steam thing to get screenshots of my battle with the credits, but Steam didn't save them because Steam. But oh well, moving on! It took me hours of retries, but I finally got through the credits unscathed.
It's finally time to see what's in store...
Two 
s, a computer, rope... butterflies... and a weird book?
Let's go counterclockwise, and hit the middle dog last.
Honestly feels like I should've expected that.
Another examination gives one more line:
Yeah.
A regular movie star, that dog.
No. No they are not.
I don't think I have any further comment on that.
So yeah, this confirms that is pretty much a self-insert. Which is amusing. So then... what's Toby doing sleeping there?
We can fight Toby? I wonder how strong this bonus boss must be. How determined.
You raised my hopes and dashed them quite expertly. Bravo.
On the other hand, if you choose to leave him alone...
And that's all the secrets behind the mysterious door in Snowdin. As far as I know.