Your instinct to check the list of special rooms was very well-founded, but what you've found is referred to as a "special level," since it's not really just a room.
What you've found appears to be the Big Room, which is generated in 40% of games, and will always appear between Dlvl 10-12 if it's present. Particularly, you seem to have found variant #3:
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As the wiki page mentions, the Big Room can be very dangerous, because the room is populated with lots of creatures and few walls, meaning that those creatures will often able to see you as soon as you enter the level, and may begin rushing towards you. Conflict and, of course, Elbereth can be very useful here.
If you entered the room through a pit or trapdoor and therefore aren't sure about where the stairs are, you may want to consider digging through the floor to escape if you're feeling uncertain; you can always come back later on better terms.
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. Is it a Mind Flayer or a Dwarf King? Who knows?