I was playing Fallout 2 today, and I stumbled across Modoc, completely by accident (I've been here before in other saves, but). And it got me thinking about the Slags. At a glance, they seem to be just Trogs (what with their being mutated and ulta-sensitivity to sunlight), but some of the Slags happen to use Ghoul sprites.

My first instinct is that some Slags are more mutated than others, and the Ghoul sprites were used to illustrate this without having to make a whole new look for such minor characters. On the other hand, they actually could just be Ghouls. But what about the non-Ghoul Slags?

The thing is, I don't believe it's ever really said what caused the Slag's mutation. If it's a similar effect to the Van Buren-style Trogs (isolation in near-darkness for generations, plus trace amounts of radiation seeping in), why weren't the Trogs simply called Slags? What's different about them?

So in conclusion, just what are Slags supposed to be, and what made them that way?

The Slags are a specfic type of Mutant, caused mostly by living their entire life in darkness. They aren't ghouls, but they're more mutated than the average human.
http://fallout.wikia.com/wiki/Slags

The wiki doesn't state anything beyond what I've said.
What makes them different enough to the Trogs to warrant a different name?

They live underground in a fiberglass tree.

Mutant jockeys! Of course! ;D

They're effectively allergic to sunlight, for one. Their eyes have mutated to allow them to see perfectly in almost total darkness. Their mutations are adaptions to the environment.

Ghouls on the other hand are created by very high radiation areas.

Maybe it's a mix of humans and ghouls mutated or should I say: "evolved" to the darkness caused by the surroundings and perhaps a steady diet of glowing mushrooms.

I suppose ghouls still carry human DNA. It doesn't matter actually because imo any creature will evolve to its surroundings or perish = nature's way, mutated or not.

They're based on H.G Wells' the Time machine's morlocks ala the 1960's film version.
Trogs at least in FO3, as that is really the only time in game we see them. Have more of a modern frog mutant / Lovecraft Innsmouth feel to them.

Edit: of note Slag is a general or specific insult in england, Its most common meaning is a promiscous devious woman..

The common way of saying it is "You / You're a SLAAGGG!!"..
As in "you sleept with my brother??? get out you SLAAAGGG!!"

Maybe you become a slag before you become a trog.
Or maybe Trogs received a more severe amount of radiation compared to slags.