I am summoned

Bit late to the party, it seems. Let's see here...

Alright, so, as noted above, the neutral evil exemplars are daemons, aka yugoloths; they acquired the latter name during the moral panic period of 2E (alongside the baatezu, tanar'ri and gehreleths). These latter three all got reversed, but whether due to the similarity between "daemon" and "demon," the fact that yugoloths acquired so much characterization under that name during the Planescape line, or just because they were the only actual "tribe" of daemon (unlike devils and demons, which encompass many types of outsider) the name stuck in Third Edition.

Ahrimanes... is a whole different subject that I've gone on about at length before.

As for Apomps the Triple Aspected, he is a god-being who dwells in the lowest layer of Carceri, the plane he settled with his repugnant creations. He was exiled by the other baernaloths for his act of creating the demodands, who now passionately hate yugoloths. Under normal conditions it is said there are 9999 gehreleths - 3333 of each caste; however, when the Blood War spikes, they double this number. This may, of course, be simple propaganda - or the desperate hope of those who wish to believe any kind of fiend could be wiped out with anything approaching reasonable ease.