I agree that Pluto works just fine as Yuggoth, as suggested in TWiD
. In fact I think it's a perfect example of HPL's genius for weaving concepts from cutting-edge scientific research into his fiction. Consider: Lovecraft started writing the story in February 1930; Pluto's discovery was announced on March 13 of that year; it took him until September to finish the story, which was published the following year. But the action in the story takes place mainly in the years 1927-28, and Lovecraft incorporated Yuggoth's ("Pluto's") 'discovery' into this story:
Their main immediate abode is a still undiscovered and almost lightless planet at the very edge of our solar system - beyond Neptune, and the ninth in distance from the sun. It is, as we have inferred, the object mystically hinted at as "Yuggoth" in certain ancient and forbidden writings; and it will soon be the scene of a strange focussing of thought upon our world in an effort to facilitate mental rapport. I would not be surprised if astronomers become sufficiently sensitive to these thought-currents to discover Yuggoth when the Outer Ones wish them to do so.
He even mentions how "hideously appropriate" it is that the planet should have been named Pluto, for the god of the underworld (the Roman version of Hades, of course).
HPL is so good at this, I love how his talk of inconceivable locations "beyond angled space" is clearly inspired by general relativity, and how the Mi-Go are made of matter that "vibrates at a different frequency" from terrestrial matter comes from quantum mechanics and what was, at the time, absolutely cutting-edge ideas in physics about the nature of atoms. Never mind all the stuff about the Earth's deep geological past. I don't think there are many science fiction authors who get quite as much real, cutting-edge science into their writing as Lovecraft did in his later stories.