The "yochi virago" was mentioned by name in the AD&D module Reverse Dungeon - two of them were accompanying a drow expedition. It's implied they were demons summoned by the priestesses but I've never seen this name mentioned anywhere else. Can anyone clarify what they are?
That section's introduction mentions in passing that yochi are handmaidens of Lloth. You're right that they're never mentioned elsewhere, but that one detail is a clue that lets us track them down.
Looking into Lloth's lore you'll find that her handmaidens are called yochlol — a curiously similar name to yochi. Looking into yochlol further in Drow of the Underdark you can see that their overall descriptions match: they have multiple forms including spider and elf, they have the right variety of ACs, etc.
So a yochi is a mistake on the writer's or editor's part, and those demonic handmaidens in the company of the drow in Reverse Dungeon are Lloth's Tanar'ri servants of choice, yochlols. I have no idea where the “virago” part comes from. Given it's in italics in “Yochi virago”, maybe the intent was for it to be a title or a sub-type of yochlol; given the baldness of the naming error itself though, I wouldn't read too much into it.
My best guess is that Reverse Dungeon had a very nasty production history, and errors like this are the scars of that history. It was published in 2000, which means it was one of the projects in-flight when TSR went abruptly bankrupt, Wizards of the Coast suddenly took over, and all the current projects were either shoved out the door as quickly as possible to fulfill obligations or dropped into a vault and never seen again. Reverse Dungeon obviously wasn't dumped into the memory hole, so it's lucky to exist, even with errors.