The Mi-go are an extraterrestrial species from the planet Yuggoth. They are described as winged creatures with large claws and heads covered in antennae. The moon on their planet shows secret symbols which the Mi-go protect, mostly by hunting down anyone who writes the symbols down. The Mi-go are a scientifically and technologically advanced race, having a particularly sophisticated grasp of surgical techniques.
The Mi-go are large, pinkish, fungoid, crustacean-like entities the size of a man with a "convoluted ellipsoid" composed of pyramided, fleshy rings and covered in antennae where a head would normally be. According to two reports in the original short story, their bodies consist of a form of matter that does not occur naturally on Earth. Interestingly, they are capable of going into suspended animation until softened and reheated by the sun or some other source of heat. They are about 5 feet (1.5 m) long, and their crustacean-like bodies bear numerous sets of paired appendages. They also possess a pair of membranous bat-like wings which are used to fly through the "ether" of outer space (a pre-Einsteinian concept). The wings do not function well on Earth. Several other races in Lovecraft's Mythos have wings like these as well.
The Mi-go can transport humans from Earth to Pluto (and beyond) and back again by removing the subject's brain and placing it into a "brain cylinder", which can be attached to external devices to allow it to see, hear, and speak.
The Mi-go previously worshipped the beings Yog-Sothoth, Nyarlathotep, and Shub-Niggurath, among others, although in newer works it is acknowledged that the Mi-go are at war with the Elder Gods. Their moral system is completely alien, making them seem highly malicious from a human perspective.
One of the moons of Yuggoth holds designs that are sacred to the Mi-go. The symbols inscribed upon the moon are useful in various processes mentioned in the Necronomicon. It is said that transcriptions of these designs can be sensed by the Mi-go, and those possessing them shall be hunted down by the few remaining on Earth.
Hastur apparently despises the Mi-go. His cult, servants of "Him Who Is Not to be Named", are dedicated to hunting them down and exterminating the fungoid threat. However, in the original story a human ally of the Mi-go mentions "Him Who is not to be Named" in the list of honored entities along with Nyarlathotep and Shub-Niggurath.
The Mi-go are fundamentally alien with respect to Earthly life, being composed of matter different than that commonly found on Earth. However, they are described as being most like fungi in terms of their biology, although their outward appearance is much like that of a crustacean.