Dungeon Masters Companion: Monster List (Other Planes)

The monster list for Other Planes in the Companion Set rules mainly involves elemental creatures, including those from the Basic and Expert Sets with elemental origins,* and others which are new

*as described in Eldritch Wizardry 

In addition to the elementals themselves (which range up to 32 Hit Dice), there are 'good guy' and 'bad guy' races on each plane.  Some of these life forms are very strange, but all are playable and interesting.

Frank Mentzer, Imagine #18 (September, 1984)

Elementals take double, normal, or no/minimal damage from attacks according to the principles of dominance and opposition, as explained in the section on Elemental Magic in Part 3: The Multiverse

The elemental principles of dominance and opposition.

Other Planes:

Aerial Servant ("Haoou") - from AD&D 1e Monster Manual

Basilisk - from D&D Expert Set

Cockatrice - from D&D Expert Set

Djinni (Lesser, Greater "Pasha") - Lesser version from D&D Expert Set

Efreeti (Lesser, Greater "Amir") - Lesser version from D&D Expert Set

Elemental - from D&D Expert Set

Gorgon - from D&D Expert Set

Helion - distant relatives of the Sun Brothers ("Sollux") from X2

Horde - life forms comprised of hundreds of insect-like bodies, enemies of kryst and undines

Hydrax - crab-like creatures made of ice, enemies of undines and kryst

Invisible Stalker ("Sshai") - from the D&D Expert Set

Kryst - urchin-like beings comprised of golden crystal spikes, enemies of hordes and hydrax

Medusa - from the D&D Basic Set

Plasm (Normal, Giant) - skeletal creatures comprised of elemental material and ectoplasm

Salamander (Flame, Frost) - from D&D Expert Set

Undine - featureless, snake-like beings made of water, enemies of hydrax and hordes