Here is a list of monster habitats I archived back when ff9hq got themselves rid of FoI. When we went independant, I made some archives of important lists, such as this one. Because it is so old, some of these won't be viable any more.

Tundras,
River basins,
Volcanic ash,
Shallows (underwater),
Deep sea,
Haunted underwater basements,
Underwater caves,
Tidal Flats,
Low/mid/high altitude,
Arctic,
Deep Caves,
Core of the world,
Lunar surface,
dream scape,
Steppe,
Lush Gaia,
Psychological ward,
valley of death,
Dried riverbed,
Senate,
Mediterranean (Homer's Odyssey),
Texas,
Places of contradiction (like the coral reefs of FF7 Forgotten city),
Petrified Forest,
Wreckage,
Fortress,
Elemental caves,
Disturbed timescape,
other worlds

Some explainations for the above:
Tundra - a harshly cold climate with resiliant life. Very hostile environment creating very cold resistant preditorial beasts. Natural skin colors provide protection from attack as well as adding stealth to movement.

Desert - a sandy and rocky terrain with little life. Very hostile environment creating heat resiliant and water preserving beasts. Colors tend to stay drab and earthy ranging from light tans to dark browns and blacks.

Coastal Plain - Moderately flat terrain declining towards the shoreline. Home to many forests and plains providing many many materials for comfotable living for most beasts. Humans, as well as most other sentient forms live here. Cities are easily built and maintained, especially near river deltas and inlets.

Mountainous - Hostility of environment increases with altitude. Mountains provide homes to many animals. Low altitude mountains have smaller beasts, while higher altitude mountains can be home to gigantic beasts of epic proportions.

Frozen Lake (surface) - very slipery terrain in which flaws in the ice may create rifts and cracks. Enough of these form from heavy footsteps can cause even the hardiest souls to drown. Though not a home to set species, many cases of different underwater elements can attack through holes and desperate monsters may search for food by crossing this environement.

Frozen Lake (underwater) - Extremely hostile environement with very few life forms of any kind. Only the most hardy beasts and fish can live in these shallows and survive. Some act as heat sources and drown unsuspecting surface beasts by melting away the ice beneath them.

List Continued...
Forest - Animated Trees, Rock Chamelions, Army Ants
Desert - Scarabs, Locusts, Scorpions, Buzzards
Tundra - Ice wolves (elementals), Crystal Hawks, Snow bandits
Glacier - Frozen Thoughts (spirits), Forgotten Beasts, Frosty Hearts
Dead space (dimensional rift zone) - Forgotten Poets, Derelicts, Lost Souls
Dreamscape (incarnates) - Aggression, Lonliness, Hatred, Fatigue, Depression
Lifespace (innerspace) - Amoeba, Spores, Sentinals, Guardians, Virus, Plague

Overworld: Clouds. basically villages on top of the clouds, or like hovering cities.
Burrows: areas where you can go underground, where a city may be located, or maybe to exterminate monsters. Basically underground.

Forest: Plentiful green trees, areas of open fields, lush warm enviroment.
Deep dark forest: Further in the forest. Darker, thick with brush, trees and over growth.
Ruins: Ruins of an old building which are usually infested by small monsters.
Fields: A large field covered with grain or plants of some kind. Rodents could be living here, and the enviornment is generally peaceful.

SUnken Fields - area of plains mired with sink holes that give away under certain weight. Dungeons and caves can reside beneath...
Riftspace - hole in the world that goes deeper than any other. Found around a mining expedition.
Dessert - miles and miles and miles of pastries? YOu bet. Ice cream glaciers to doughnut stones to Jello flats, this place is loaded with every dessert imaginable... Monsters - toothaches

Space:
Any battles that will take place in space? The battle could be set near stars or near a black hole.