Yuggoth is the name used by various Mythos beings and scholars of the Mythos to refer to an outer planet in the Solar System. It is generally believed that Yuggoth is the same world known to humans as Pluto.
H. P. Lovecraft's story "The Whisperer in Darkness" introduced Yuggoth to the Mythos. Pluto was actually discovered while the story was being written, making it one of the first works of fiction to include the new planet.
However, conditions on Yuggoth are quite different from those that modern science suggest exist on Pluto. Black tarry rivers flow through the ruins of an ancient civilization, and the planet's surface is inhabited by an outpost of Mi-go, a species of crustacean-like, fungal aliens.
Ghatanothoa traveled from Yuggoth to the continent of Mu on Earth along with its worshipers.
Clark Ashton Smith, in "The Family Tree of the Gods", said that Hzioulquoigmnzhah, Tsathoggua, Cxaxukluth, and several other Great Old Ones lived on Yuggoth for a time; Cxaxukluth still remains there.
In Richard A. Lupoff's "The Discovery of the Ghooric Zone", Yuggoth is not Pluto, but a larger world even further from the Sun. It is described as twice the mass of Jupiter and possessing four moons - Nithon, Zaman, Thog, and Thok - with the latter moon inhabited by shoggoths.