A question on that, then. Are the demons the same for the world every time? I might do some test-drills to see if there's a decent enemy, and if they're all made of salt and steam then I'll just make a new world and try again.
Nope, they're procedurally generated and if there are more than say.. 3 types? They vary by embark location, as well.
You can see what types there are in the sites_and_pops report, near the bottom. However, the names are only partially descriptive. The most.. reasonable? of them are the ones that are dinosaur-ish. At least those you can kill with normal weapons.
Often, however, they are made of salt, steam, or similar materials if there just a few types. Here's an example from setting the number to 8 (yes, I know there are only six, that's the way it goes)
Unnumbered one-eyed monsters
Unnumbered mole fiends
Unnumbered one-eyed demons
Unnumbered demons of steam
Unnumbered monsters of steam
Unnumbered ceratopsid fiendsNow, the one-eyed monsters could be just about anything, but the ceratopsid fiends are reasonable, and if you do manage to kill them, you can also turn them into roasts/meals. Probably true of the mole fiend as well. The steam ones, well, they'll die easy, but they're not much use after the fact, typically.
It's also worth noting that even though they are Unnumbered, I have run into situations where, after the initial rush of several dozen, very few or none show up for years, after that.