Before I delve into the oinoloth, I want to settle an issue regarding yugoloths—or at least, regarding my interpretation of yugoloths. The issue involves the question of what plane yugoloths are native to, and specifically, whether they can be killed (as opposed to just destroyed) on any plane other than Gehenna, the outer plane of “lawful evil neutrals.” My take, which differs from pure canon, is that yugoloths may be numerous in Gehenna, and some yugoloths may be native to that plane, but Hades has as strong a claim on them, if not stronger.
The fifth-edition Monster Manual says:
Back to Gehenna. When a yugoloth dies, it dissolves into a pool of ichor and reforms at full strength on the Bleak Eternity of Gehenna. Only on its native plane can a yugoloth be destroyed permanently. A yugoloth knows this and acts accordingly. When summoned to other planes, a yugoloth fights without concern for its own well-being. On Gehenna, it is more apt to retreat or plead for mercy if its demise seems imminent.
This paragraph isn’t as ironclad a statement that yugoloths are native to Gehenna, and only to Gehenna, as one might think. First, it doesn’t state explicitly what a yugoloth’s native plane is, only that if it’s killed somewhere other than its native plane, it re-forms in Gehenna. Another paragraph on the same page states, “Yugoloths are fickle fiends that inhabit the planes of Acheron, Gehenna, Hades, and Carceri” (the last of these, in AD&D, originally called “Tarterus,” a misspelling of “Tartarus”), implying that any of these planes could be a yugoloth’s native plane. Second, I reserve the right to declare occasionally that the Monster Manual flavor text is full of it, as in the case of the soldierly hobgoblin that for some reason instantly forgets all its training and abandons all its discipline if it happens to catch a glimpse of an elf, or the use of “efreeti” as a singular noun rather than “efreet.”
Before yugoloths were yugoloths, they were “daemons,” the neutral evil counterpart to lawful evil devils and chaotic evil demons. The first daemons to appear in a D&D sourcebook were the guardian daemon, mezzodaemon and nycadaemon in the Fiend Folio (the last two are now the mezzoloth and nycaloth). The guardian daemon’s home plane is unspecified, but the mezzodaemon and nycadaemon are described as inhabiting “the Lower Planes between the Abyssal Layers and the Hells—i.e., Tarterus, Hades, Gehenna” (idiosyncratic italics in original). Gehenna doesn’t even appear first in that list.
All daemons/yugoloths are neutral evil—always have been, probably always will be. And in the Outer Planes cosmology, the plane corresponding to the neutral evil alignment is Hades. If there’s any plane likely to have spawned a neutral species of fiend that sells its services to both of the feuding sides in the Blood War, it’s Hades. And for that matter, if daemons are found in Tartarus (C/NE), Hades (NE) and Gehenna (L/NE), chances are, they started in Hades and spread outward from there. (In fact, the second-edition Planeswalker’s Handbook, says, “The yugoloths live on Gehenna, although some claim the Gray Waste [Hades] was their original case [sic].”) In Planescape and more recent cosmology, if yugoloths have made it as far as Acheron, then perhaps they did once originate in Gehenna, but there’s still no reason why they shouldn’t feel just as much at home in Hades, just as Polish-Americans born in Chicago to Polish parents may be native to the United States but still think of themselves as Poles.
And finally, Dungeon Masters get to make this stuff work however they want to. If you want to insist that all yugoloths are native to Gehenna and only to Gehenna, can always be killed there and can never be killed anywhere else, that’s your call. If you want to say that there exist yugoloths native to Gehenna and also yugoloths native to Hades (or Carceri or Acheron), and that any of them can be killed on their own home plane, that’s also your call. If you want to say, as I like to, that yugoloths are actually native to Hades but have managed to colonize Gehenna and make it their own, resulting in their being vulnerable on both planes, that’s your call, too! All of this is made-up, and you’re at liberty to make stuff up as well.
However you set your cosmology up, the most important thing is that yugoloths encountered on the prime material plane—where most PCs spend most of their lives—can’t be permanently killed there, and consequently aren’t going to be concerned about self-preservation the way, say, a bear is. When your PCs are high enough level to go plane-hopping, then you can deal with the fine print.