Razz said:
FCI was about demons, whether it's obyriths, loumara, or tanar'ri, they were still demons.
"Demons" is an almost meaningless term. It just means they're chaotic evil fiends. "Devil" only means lawful evil fiends. Loumaras aren't remotely related to tanar'ri, for example.
By those loose standards, night hags, diakka, gehreleths, barghests, avari, hordlings and so on are all the same "race" as yugoloths. Of course they're not, because "NE fiend" isn't a race. But "demon" and "devil" aren't races either.
The problem is that because neutral evil fiends don't have a general name (by analogy, it would be "daemons"), you're applying to them a different standard.
Whether they're night hags, hordlings, yugoloths, or nightmares, they're all "daemons."
There are two main categories of "daemons," the spawn of the progenitors of evil (yugoloths and demodands) and the spawn of mortal souls (larvae, shades, night hags, and hordlings - arguably barghests also count because they drain life energy to mature). These two catagories are analogous to obyriths and tanar'ri. Both of these groups inhabit the same areas and their politics are fatally intertwined. Each should get a chapter in the same book.
FCII covered very few nondevils. In fact, only Kalabons and Hellfire Engines weren't devils/baatezu, everything else was.
You're acting as if devils and baatezu are synonyms. They're not.
Imps and kytons aren't baatezu, nor are soulshells or hellbred, nor are ayperobos. The Hag Countess wasn't a baatezu. Both erinyes and abishai, though they have the baatezu subtype, have seperate origins from baatezu, and are as different from baatezu as obyriths are from tanar'ri.
Claiming that the FCIII should only cover yugoloths is no different from insisting that only baatezu be covered in the FCII or only tanar'ri should be covered in the FCI.