As PakaNoHida said, also sometimes they will chase enemies into other biomes.

The border areas of all biomes are free for all matches between mobs, you may have been caught near such activity.

I had a 1 pixel plains biome in Black Forest spawn a deathsquito, a fuling, and a lox in my current game. I ran away but the 'squito got me anyway lol.

You probably have a small spit of Plains somewhere with a spawn point. I've had strips of plains wedged in near meadows or swamp with the occasional deathsquito or fuling.

Only other possibility would be a fuling going on a rampage and being constantly aggro'd by black forest denizens. I've had that occur before.

I've come across fulings in both the Swamp & the Black Forest. In both cases those biomes shared a border with the Plains.

I've had Fulings come after me in the BF Biome, but plains were always nearby, so they do cross the border. This was on a solo map.

Encountered them there at night on a few occasions recently, so I guess it's normal now.

no MODS, no Yagluth trophy, no Bonemass trophy, only Moder, The Elder, and ElectroStag defeated, lol...

It must be a new random encounter, it is usually just a single unit; nothing major but still unexpected after so many hours played and it's outside my main base which has been cleared around and scavved by me on a regular basis for Thistle and Blue Berries and of course, Copper nodes aplenty. It's a good spot for a base, on the water, in a Black Forest. Now a new 'evening' encounter...

I ran into them in the swamp once. They were in one of their wooden houses. I figure it's the same as you run into draugers in ruins in the meadow sometimes. Rare, but happens.

Originally posted by rexpiscator:

no MODS, no Yagluth trophy, no Bonemass trophy, only Moder, The Elder, and ElectroStag defeated, lol...

Defeating Moder triggers the fuling raids. It used to be only defeating Yag that made them spawn in meadows without a raid, but this may have changed.

They wander out of the Plains as they attack nearby creatures

Never seen one outside of Plains myself. Are you sure there isn't like one pixel of Plains there or nearby? "Nano biomes" sometimes occur.

Then again, I once saw a Surtling spawner in what was labeled as Black Forest with no Swamp anywhere near...

This is a new encounter in a very old area. He seems to hover around a dungeon.... It's an entire Black Forest island...

Day 1200+ and this fuling in the BF is very new, after the last update.

Killed him twice now, so he appears to be respawning...never seen this before, fuling in BF.

If you’ve placed yagluth’s trophy on the pedestal at spawn, fulings can spawn in almost every biome at night. This might pertain if you were playing on a multiplayer server.

Otherwise fuling only spawn in plains biomes, but do tend to wander into nearby biomes as they will fight literally any non-plains enemy and can detect them from quite a distance. They were likely fighting the skeletons by the dungeon in that case.

Any mods?

That behaviour is what skeletons outside dungeons have.
Have you checked the large map, zoomed in all the way, that there is no Plains there?