I would like to discuss the known origins of various Demons.
Most Demon types are corrupted Mortal Races yet the Mo’arg, Nathrezim, Pit Lords, Abyssals, Infernals, Felbats and Void Hounds are all native to the Twisting Nether.
Infernal
Of the natural Demons it seems Infernals are constructs infused with Fel Elementals(ones created anywhere but Xoroth) and Fire Elementals(the ones from Xoroth) with the rare Void and Ice Elementals.
Pit Lords
Pit Lords seem to have originated as Infernals or Abyssals if Goroth is any indication.
Mo’arg
Aggonis son of the Pit Lord Aggonar is a Mo’arg showing that Mo’arg are descendants of Pit Lords.
Seems the more brutish Demons native to the Twisting Nether are all descended from Fel Elementals shoved into rocks by(according to Chronicle) powerful and intelligent Demons I.E. Nathrezim!
Dreadlords
Antaens were introduced to Players as Argus Dreadlords in the concept art so they seem to be a breed of Nathrezim.
Void Hounds
While many are native to the Twisting Nether and thus breathe Fel others are native to the Void and thus breathe Void.
Now onto the other Demons:
Ered’ruin
Ered’ruin Doomguard were first used by Sargeras to sniff out Fel users before he released and welcomed them into the Burning Legion.
Eredar
Eredar were turned into Wrathguard(who have the faces of Broken), Man’ari, Eredar Brutes and Eredar Doomguard(which consist of Kil’jaeden and the Doommaidens). Man’ari, Eredar Brutes and Eredar Doomguard seem(confirmed in the Brutes’ case) to be a deliberate transformation while Wrathguards and the preceding Krokul/Broken Eredar are the usual transformation due to Fel Magic.
Satyrs and Terrorguard
Satyrs and Terrorguard seem be Mortals transformed by a special ritual with purple Terrorguard like Verinias the Twisted being Night Elves.
Aranasi
Aranasi appear to be a corrupted Race from some conquered Legion world possibly Rancora.
Wyrmtongue
According to Treasure Master Iks’reeged he spawned within the deepest recesses of the Twisting Nether so Wyrmtongue either have breeders within the depths of the Nether or are natural Demons.
Observers
Observers originated from the Great Dark Beyond and thus are a corrupted race.
Inquisitors
Inquisitors are the most mysterious Demon in WoW according to Inquisitor Tormentorum.
Imps
Imps originate from Azeroth according to https://wow.gamepedia.com/Daglop#Quotes
.
Overfiends and Fel Core Hounds
Overfiends draw power from the Firelands according to https://wow.gamepedia.com/Conflagration_(quest)#Description
and thus likely originate from the Firelands itself as would Imps(which appear in the Firelands) and Fel Core Hounds.
Succubi
Female Night Elves seem to turn into Succubi as shown by Sensiria. Male Night Elves wind up like Illidan.
Felhounds
Created from the nightmares of a mad god(I.E. Sargeras himself) they are Sargeras’s pets.
Felskorn
Vrykul seem to become Demons without needing to transform physically though they obviously can physically change as seen with Skovald.
Feltotem
They were once Bloodtotem though since they are Demons they are now bound to the Twisting Nether and will reform in time.
Feltalon
Demon Harpies. They look like Harpies with Fel energies flowing from their wings.
Shivarra and Gorefiend
Griselda the Crone was an Orc turned into a Shivarra due to heavy Fel infusion by Gul’dan indicating that Shivarra are Female Orcs(funny how Sargeras decided to use Female Orcs as corrupters of the Titans once he obtained Aggramar). The only Male Orc Demon encountered is Gorefiend.
Thoughts on all this?