The manes (sub-demon) first appeared under the "demon" heading in the original first edition Monster Manual (1977).
The manes (least tanar'ri) appear in second edition in the Monstrous Compendium Volume Outer Planes Appendix (1991) under the "tanar'ri" heading, and are also detailed under the "tanar'ri" heading in the first Planescape Monstrous Compendium Appendix (1994).
Manes resemble little fat deformed men, and have no will of their own. Thanks to their recklessness they are used by more powerful demons as the simplest soldiers of the Abyss: they build up the core of the army of every demon lord and they are also the first demons to be sacrificed, whenever necessary. In large numbers they can be a dangerous enemy.