I had to go googling to see if I could find a list of what was in the MC1 and what was in the MC2 (my own MC is mixed together because MC1 and MC2 were I think the only ones where the intermingling of pages actually worked with no monsters needing to slide in between two monsters printed back-to-back).
Found these lists:
List of Advanced Dungeons & Dragons 2nd edition monsters
Just glancing down from the top I see in the MC2 the Basilisk, the Bulette, the Doppelganger, the Dwarf, the Drow, the Ettin, the Cyclops, the Griffin, all of the Hags, the Hell Hound, the Mimic, the Rust Monster, the Shambling Mound, and the Stirge among many others that I would now consider "core" monsters but might not have at the time.
Right - the MM2 was "here are a bunch of new monsters". But if you wanted a 2e replacement for the 1e Monster Manual with almost all of the stats in it you had to get the MC1 and the MC2. Eventually TSR either realized their mistake or the MCs got expensive to produce because they switched away from looseleafs and to actual books, and when they did IIRC the hardcover Monstrous Compendium basically had the creatures from MC1 and MC2 in it.