Abominations were added later. Every zone now has a physically big high health, hard hitting slow guy not counting the meadows. The plains may have 2, depending on how you feel about wildlife.
The wraith comes out at night only, and you should wait a while before hanging out there at night.

How to handle it.. hoe a path is a good plan. Leeches are not that dangerous but swimming in deep water with 20 of them is indeed a bad plan AND they will chew on your boat AND the like to have stars (or maybe its large quantity = more starred about).

the tried and true approach to the swamp that works for me:
find a safe way to take a peek at night without going in. Circle it but stay in a lower biome, and look for the bright green torches. Mark the estimated spot on your map via crud line of site triangulation. Come back in the day and check out the sites you marked. Find a crypt and get into it asap, even if you have to train half the swamp to the door. Once inside, the fights will be 1x1 mostly and almost completely under your control, and even rangeable. Collect your 30 pieces of iron and run back. Repeat a couple times. Go make better stuff, and it becomes as easy as anything else does vs harder stuff. If you feel confident in your ability to grab crypts and stay out of trouble, an iron pick is a must have first item. If not, shield/weapon first.