What Pants said regarding demons and devils.
I'd add that among Tanar'ri, the point where a Balor might transcend their status (or another true tanar'ri might even possibly bypass balor to become unique), is often but not always the same time that they attempt to force their will upon a layer of the Abyss itself. This apotheosis creates something of a blur between Abyssal Lord and layer of the Abyss, and you can't always make clear where one ends and the other begins, except that in this nominal symbiosis of lord and layer, the lord has to be able to enforce their will upon a sometimes unwilling, and hungry, subject.
For yugoloths there are multiple paths to a unique form, and what unique actually is, is sometimes hard to say with yugoloths given their racial obsession with purity of form and substance. The General of Gehenna might not necessarily be so unique, but he exists as the first and most powerful Ultroloth in the multiverse, but since that time he might have shifted form by his own power, or been given such a form by the Baernaloths of The Demented to differentiate him as the highest recipient of their favor and wisdom.
Oinoloth as a position and title carries with it the blessing/curse of a unique form. The throne of Khin-Oin grants its holder obscene power yes, but it inflicts them with a unique, individualized curse such as the rotting, sloughing flesh that haunted Anthraxus, and the as yet undescribed "gift" that Mydianchlarus gained for his success at forcing that prior Oinoloth to voluntarily give up the throne. Perhaps Mydianchlarus gave him some sort of insight into the position itself, something that made Anthraxus horrified at keeping the position. Perhaps the rotting that Anthraxus endured was not a general trait given by the Siege Malicious, but a rather pointed expression of unhappiness by the Tower, or the 'loths' makers at a perceived lack of purity on the part of Anthraxus when he took the position.
Mydianchlarus and the General of Gehenna are 'unique' yugoloths, and it's possible that you might also include Helekanalaith the Keeper of the Tower Arcane (though he might just be an obscenely powerful arcanaloth), and Larsdana Ap Neut (same thing as Helekanalaith).
A subclass of unique yugoloths are called Altraloths, and their unique forms come from a pact they make with powerful cabals of Night Hags, a process that can be lethal. The 'loth can gain power at the expense of position within the orthodox heirarchy of their race, something which doesn't necessarily sit well with the other 'loths, but which the gain in personal power typically balances. Charon, Bubonix, Taba, Xenghara, Cholerix, and Anthraxus are the known members of this group.
And then the various Baernaloths are in a class all their own, and they're not technically yugoloths.
