The Monster List in the Companion Set rules was divided into two parts: the Prime Plane (Apparition to Will o' Wisp*) and Other Planes (Aerial Servant to Undine).
The creatures include Beholders and Blast Spores, larger versions of Dragons, a couple of brand new monsters (Drolem, Gargantua), nine powerful new Undead, several new monsters from previous modules (X2, X4, X5, X7, M1, M2), and a few from Cook/Marsh that didn't make it into Mentzer Expert.
Illustration by Jeff Easley
Prime Plane:
Apparition - a form of Undead (see Phantom)
Banshee - a form of Undead (see Haunt); the earlier version from B4 was designated as a "Lesser" Banshee in AC9
Beholder (from Greyhawk)
Blast Spore (from AD&D 1e Monster Manual)
Dolphin (from Blackmoor; X7)
Dragon - Large and Huge versions of those in the D&D Basic Set
Dragon Turtle - from OD&D vol. 3/AD&D 1e Monster Manual; Cook/Marsh
Drolem - a dragon-golem
Druj - a form of Undead (see Spirit)
Gargantua (Carrion Crawler, Gargoyle, Troll) - huge versions of smaller monsters, the products of research of the mad wizard Gargantua (mentioned in CM1)
Ghost - a form of Undead (see Haunt)
Golem, Mud (from X2)
Golem, Obsidian (from M2)
Grab Grass (from X2)
Gremlin (from X2)
Haunt (Banshee, Ghost, Poltergeist); the earlier version from B7 was designated as a "Lesser" Haunt in AC9
Malfera (from X5)
Manscorpion (from X4)
Manta Ray (Normal, Giant) - from Blackmoor; X7
Mujina (from X5)
Odic - a form of Undead (see Spirit)
Phantom (Apparition, Shade, Vision)
Poltergeist - a form of Undead (see Haunt)
Revenant - a form of Undead (see Spirit)
Rock or Cave Toad (from M1)
Shade - a form of Undead (see Phantom)
Shark (Bull, Mako, Great White) - from Cook/Marsh; similar to Sharks in the AD&D 1e Monster Manual; Wereshark (from Holmes)
Snow Ape (from M1)
Spectral Hound (from X5)
Spirit (Druj, Odic, Revenant)
Vision - a form of Undead (see Phantom)
Weasel, Giant - from Greyhawk/AD&D 1e Monster Manual; Cook/Marsh
Whales (Killer, Narwhal, Sperm) - from Cook/Marsh; - similar to Whales in the AD&D 1e Monster Manual