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...