Back in the late 2000s Stephen C. Klauk wrote two bestiaries, Bestiary Malfearous and Bestiary Nefarious, for his decades-old homebrew campaign setting Amberos. In addition to a lot of original monsters created for his setting, it includes a number of substitutes for WotC IP monsters. These include the serpentine aspii in Bestiary Nefarious and the octopoid menten in Bestiary Malfearous, who are described as mortal enemies in their similar quests to remake the world in their own image. Nice.
The two books are also each several hundred pages long. Damn. If only I were so creative and dedicated...
Anyway, the aspii are immortal serpentfolk that use alchemy to transform others into hybrid slaves, and the menten are octopoid aliens from the Realm of Madness that feed on minds (not physical brains, but the energy of the mind itself).
The aspii are similar to other serpentfolk I've covered previously, and share traits with all of them. The true aspii are giant semihumanoid cobras (similar to the illujan lords and anguineum ophiduans) with snakes for arms (similar to the inphidians from the Tome of Horrors series). They create hybrid slaves through both alchemical transmutation and more conventional crossbreeding.
The menten are semihumanoid octopoids that use modified tentacles are arms and legs. They were created by someone else to conquer the material plane by devouring or converting the inhabitants. There are several degrees of slavery, including being mind-wiped, used as raw material to make a horrifying "flesh engine", or taken to the Realm of Madness and transformed into "thrallkin" who form the lowest rank of the true menten species.
Unfortunately, aspii and menten don't seem to be Open Game Content. Not that I mind considering the other substitutes available, but the bit about them having similar goals and therefore being mortal enemies was actually pretty clever and I'm surprised nobody else thought it up. I'm definitely borrowing that bit.