Do the true pacifist run and go through every single part of the credits so then you must wait for the Special Thanks and dodge all the credits. If done right after the Special Thanks it should say: A door has opened. or something along those lines. Now head back to Snowdin and mainly go back to the room after that puzzle with the bridge (Not the gauntlet of doom or whatever it is) and head through the mysterious hallway and find the dog's door. Go through it and you've found the Dog's Room! Leave a like if this guide helped you.