Well here is another stumper, I'm wondering if this list was created with D&D 3 in mind? I did however notice this on the OSR Google + "Martin Thomas posted the following... For those "not-in-the-know" an "outsider" is anything from the "outer planes" - so stuff like elementals, solars, planetars, devils, demons, and my "favorite"...
To quote Joshua De Santo http://geniuslocigames.blogspot.ca/2013/09/30-day-challenge-day-18-them.html "As the days wear on and the challenge grows I am starting to see that I am woefully under-prepared for some of the questions"
I'm having a lot of difficulty as well with some of these questions. I haven't played a whole lot of Space / Planescape RPG stuff, so this was a bit of a difficult one. I've decided to go with the Glabrezu, and not because I've ever had one in any of my campaigns, but because I always wanted to. There is something about flipping thru the AD&D 2nd Edition monster manual, that just makes you go "Gawd damn, we gotta roll up some HIGH level characters, cuz I want to throw this and that at you guys". Same goes for the Tarrasque (which I'm sure will appear on day 22). Here is some info on the Glabrezu from the Forgotten Realms wiki.
Glabrezu are one of the most powerful types of demonictanar'ri. A glabrezu tempts its victims into ruin by luring them with power or wealth. Their imposing stature radiates authority and control, which they use to sway mortals into accepting their gifts...for a price. They use lies and deception to destroy the lives of all whom these demons meet.