The Mi-Go or Fungi From Yuggoth are an alien race created by H.P. Lovecraft that have been mentioned as being in conflict with the Brothers of the Yellow Sign, although it seems that they may also worship Hastur. They either originated upon or are based upon Yuggoth, which may be Pluto or a planet as yet unknown. They have human allies on Earth. Lovecraft associated them with stories of the Abominable Snowman (Mi-Go coming from "migou", being the Tibetan for 'Apeman').
They are the titular Star Spawn in The Old One & The Star Spawn.
They may be mentioned in The Colour Of Death as "the crustacean races of Aldebaran", and are depicted as being at war with a race that may be the Byakhee, another race, servants of Hastur, also occasionally said to be found on Yuggoth. Delta Green RPG and Delta Green: Eyes Only provide an expanded take on the Mi-Go in The Call of Cthulhu RPG in which they are served by the Greys of Conspiracy Theory lore and are manipulating Majestic 12.
The Mi-Go are usually said to serve none of the Great Old Ones, although it has been inferred that they are working with Nyarlathotep in The Whisperer in Darkness. They do, however, seem to worship various Great Old Ones to some extent, including Nyarlathotep, Shub-Niggurath and, possibly, Hastur.
In Allan and the Sundered Veil, they are identified as the Morlocks.
In Lord Rutsah's games the Mi-Go are systematically hunted down by the Byakhee to the point that their race faces extinction, often offering their help to forces opposing Hastur as a result.
See the Talk page and Truth and the Mythos for further discussion of their origin and the conflict.