First of all, Ground Spitters. Looks like only two spawners for them will exist in the game, making Ground Spitters the game's first rare monster. I have suggested rares before in the form of Bears and Woolly Mammoths, and since it seems you're following through with my idea of rare monsters through Ground Spitters, its high time I had the Ground Spitters make do with their rarity.
Why are Ground Spitters considered a rare monster? Well, in both areas they're in (Teplar Village and Muck Swamp), there are only one Ground Spitter Spawner, and they only spawn in one Ground Spitter at any given moment, not spawning any more until the Ground Spitter is killed. In order to balance out their rarity, I shall suggest giving the Ground Spitter a few more drops.:
1. Archer Shirt (Sells for $56)
2. Archer Hat (Sells for $56)
3. Wood Bow (Sells for $148)
4. Long Bow (Sells for $481)
That should help balance out their rarity.
Now, for the real swamp stuff.
First off, I made an environmental creature just like the butterfly: a frog.

The frog shown here is 4x its actual size, but yeah, I made four frames of animation for the frog leaping, hopping, whatever you wanna call it.
Next up, I made a bog monster for the swamp.:

Its design was inspired by, well, Amy from Total Drama Pahkitew Island that one time she returned to the island as a swamp creature:

Gotta give a chick mad props for walking for hours underwater like some reverse Jesus Christ. Anyway, I had split up the bog monster into all of its part in its original size (72 ppi):

Why is it that my art has a better idea on how front and back view arms work than the game does, anyhow, especially with the crabs? Anyway, I would imagine that the bog monster would be a slow creature, so I think it should have two attacks: a ranged attack where it lobs mud balls at you and a melee attack where it just flails at you in a "smashing" motion. It'd only make sense.
That aside, while guns, bows, wands, and staves seem to be in check, I think melee weapons should be touched up in order to allow warriors to catch up. Well, it's less modifying and more adding, but you catch my drift. I have made two weapons to handle the balance issue: an axe and a machete.:

Despite their size difference, the axe is a two-handed weapon (since that's a logging axe) while the machete is a one-handed weapon (due to its use for cutting through vegetation in dense forests and swamps). I based their handedness off their functionality. Mhmm.
But yeah, I think that about does it here. What do you think?