It's basically (1) variants of MM/Volo's/Mordenkainen's monsters - weaker or stronger, or with variant abilities/powers, and (2) monsters from previous editions that haven't been updated to 5e yet (mostly in 2 and, apparently, 3).Wait wait wait wait hold up: Are you trying to tell me that the Monster Manuel Expanded series of books isn't just a 3PP monster manual take full of author created/made up homebrew monsters, and is instead basically taking various monsters from older DND books and updating em/bringing them over to 5E????
looks at the Archons images
HOLY CRAP!!!!! I had no idea that is what they were. And I'm such a 3.0/3.5 element slut when it comes to having 3.0/3.5 stuff in my 5E games lore/monster/name/etc wise.