Since the engine of the game is D&D 5e, you can use any creature from the Monster Manual. Be judicious though, 5e is designed for greater magic use and more powerful characters. Not everything will be appropriate.
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Yes, I would advise avoiding anything like the Tarrasque. And the D&D Balor is basically a Balrog and very much based on Durin's Bane (though a Pit Fiend might work almost as well).
Suggestions from Appendix A (Miscellaneous Creatures) to the
Monster Manual:
- Ape. Harad or Rhun
- Awakened Tree = Huorn
- Awakened Shrub = Lesser Huorn?
- Axe Beak. Far Harad? Or maybe east of the Orocarni.
- Baboon. Harad.
- Crocodile. Harad or Rhun.
- Dire Wolf = White Wolves of the North (add
Cold Resistance and
Fear of fire) or leave as-is for anywhere else.
- Giant Bat. Misty Mountains or Grey Mountains.
- Giant Hyena. Maybe Rhun or Far Harad. A good substitute for the Wargs from Peter Jackson's
The Two Towers.
- Giant Octopus. Something that might be encountered if or when we get marine journey rules.
- Giant Shark. See "Giant Octopus".
- Giant Vulture. Might inhabit the Mountains of Mordor or ranges in Harad or Rhun.
- Lion. Found in Harad or tweak for a Cave Lion that might be found as far afield as the Blue Mountains or the Orocarni.
- Polar Bear. The Northern Waste/Forochel.
- Rhinoceros. Harad.
- Tiger. Rhun.
- Winter Wolf. Another possible creature to adapt for the White Wolf, but I would eliminate the action
Cold Breath.
The
Monster Manual also gives you a wide variety of Dragons, but I would be careful about managing their power levels and abilities. Treants are basically Ents. And there are the Dire versions of many creatures to be found in the tome. Many of the monsters like Orcs and Trolls are very different in Middle-earth, but you already have stat blocks for the most appropriate ones.