Taking a crack at listing all the monsters shown.
So for the front cover, we seem to have a Demilich (?), a Kraken, a Gelatinous Cube, a Beholder, an Ogre, a Displacer Beast, a Mimic, some bats that could be shapechanged Vampires (?), and some Hobgoblins.
On the back cover we have a Nothic, a Harpy, two smaller Beholders or Beholder-Kin (?), a Green Hag, a Gnoll, a Kobold, a Modron, a Goblin, a Zombie, a few Myconids, an Intellect Devourer, a Mind Flayer, an Owlbear, a Balor (?), the Tentacles could be another smaller Kraken (?), and if I were a betting gal I'd say the big eye in the middle is from a Green Dragon.
Overall I really like this cover and the variety of well known monsters displayed.
Edit: Of further inspection, I believe the the "Demilich" in actually a Flameskull.