From your welcome topic, i'll take this as rm2k3, but really you should write the maker on the topic. In Rpg Maker, when you make a map, you can make Areas to set monster encounters in, or just set them to appear anywhere on the map with the enemy encounter box in the map edit window.
To make an area, right click on the map, create area. Then click boundaries and set one. In the monster encounters, add some area-specific monsters.
If you want to use some monsters only on certain terrains, it the database > Monster Groups, you can check or uncheck terrains. Use the terrain and tileset tabs to set up your terrains and where each terrain is.