If you look at the 1e monster manual, you may notice something interesting about the Type I-VI demons: the names applied to them (Vrock, Hezrou, Glabrezu, Nalfeshnee, Marilith, and Balor) were meant (explicitly in the type IV and above, and implicitly in the types I-III) as individual names, but later on the names on the list were applied to each demon type as a whole.
Which is where I come in. If a Vrock is a spiky vulture man demon, what other sorts of Type I demons could there be? I'm going to take the Gorgonmilk approach over the next few weeks: together we'll describe 13 demons of each type. One from Gary, a few from me, and the rest from you, my readers. The only rules that apply are that they should reasonably be able to correspond to the abilities of the demon as featured in the Monster Manual. One or two little changes are fine, but most of the statblock should remain useful.