A forest dungeon is full of many enemies that are unique to it. You can’t go wrong with using some of the many beasts you see in a forest to place in the dungeon. If you like, you can put an unique spin on them by maybe giving them an unusual appearance or new form of attack. This gives you a great opportunity to come up with monsters that leave a lasting impression for your audience.
Bugs are the first thing that you associate with a forest. Bees, spiders, and centipedes are excellent insects to sprinkle in a forest dungeon. Even better, make them larger than life like the Skulltulas in The Legend of Zelda. Fighting oversized insects who want to eat you for breakfast takes a while to forget. One advantage about putting large bugs in a forest dungeon is you don’t have to have them on the floor, you can put them on the wall or the ceiling!
As to be expected, plant-based enemies dominate a forest dungeon. Due to their nature, most of them are stationery, meaning you only ever fight one in one spot, though there are some plants capable of moving around. They all like to bite and usually their mouth is drenched in poison. Another way they attack is to spit seeds from afar. True to their name, plants are found mostly on the floor with only very few on the wall and ceiling.
If you’re going with a haunted forest dungeon, you have to have ghosts! Otherwise, what’s the point? These phantoms strike fear in the heart as they float around from room to room, moaning and groaning, always searching for someone whose soul they can devour.
For a forest cave dungeon, sure you could go with using bats, since they’re an important part of the cave ecosystem. What if you took advantage of the darkness of the cave to put monsters who thrive in it, beasts like orcs or goblins? Wolves work well here as well.
Last and not the least, every dungeon has a boss. For the forest dungeon, the boss comes in several varieties, which is heavily cognizant on what kind the dungeon is. If it’s a tree-based one, either a large vicious plant or savage insect will suffice. For the haunted one, go with a ghost or another type of spirit. A troll, massive bat, or a huge worm fits right in the forest cave.