I am wondering is if there is a way to make a certain encounter appear when in a different part of the world map.

For example i have a goblin encounter, I only want the hero to encounter them when on the grass plains. I have a forest goblin where i want the hero to encounter them in the forest.

Any ideas or suggestions?

rightclick the map name, ...area.

RMVX doesn't work like RM2k/3 with terrain settings. There are assorted terrain scripts available, however; Search for them.

Just like he says.
In the bottom left hand corner of the editor where the maps are...
Right click the world map in your game and find "new areas" on the popup menu.
Simply encompass your forest in as many areas nessesary and make sure when you create an area in the forest,
you set encounters to your Forest Goblins instead of the plains ones.