The Dunwich Horror expansion comes with an additional Mask monster, the Beast. When to use? The Nyarlathotep sheet clearly states "add the 5 Mask monsters to the cup"...

Yeah, the Nyarlathotep sheet was created when you had only 5. Feel free to read it as "add ALL Mask monsters to the cup" Not so sure if there's an official errata, but that's the way everyone has been playing since Dunwich arrived in stores for the first time (other Masks were added in other expansions as well)

Julia's got it: use them all. If you take careful count, you'll notice that each expansion gives about enough Mask monsters to keep the ratio of Masks to non-Masks about the same. With the exception of anticipating adding more Masks from expansions, FFG knew what they were doing.

Also, worth noticing, that in the end we have exactly 1 Mask for each dimensional symbol existing (if I'm not wrong, it's a long time since I played Nyarly)

Ok, thanks!