Not Quite Canon: MI GO for D&D adjacent.

I did this - once upon a time.
I think I painted it with a wooden skewer on (what looks like) a bit of a box.

I like the Mi Go, and I think that they should be able to turn up in pretty much any setting or genre that doesn't specifically exclude them.

These are 'common' Mi Go - they come from 'Out There' but are not ultraterrestrial horrors from an anti-matter universe.

Converting from Call of Cthulhu 5e, we get the following D&D-adjacent Mi Go:

AC +2     HD 2     Mv. 75% Normal Human (Flying 125% Normal Human or 75% Bat)     ML 8

• 1 nipper attack for d6 hits, or by weapon, or grapple (then drop you from a great height)
• the AC bonus is for a high average Dex, and they can wear armour 
• average Int is 13, with a 25% chance of being able to cast d3 spells (or same % of 1-3 caster levels, though that's much the same)
• any Mi Go weapon (including all but the most straightforward hand weapons) has a 40% chance of being used incorrectly by a human - 50/50 it's aimed at the wielder/ aimed at the floor
• they are unaffected by silence/deafness and darkness/blindness because of their weird eyeless heads - you might choose to extend this to all visual and aural effects, including illusions
• my gut tells me that Mi Go are familiar with psionics, but might not be psionicists - they might even be completely immune and unable to use them

This is a fairly typical conversion profile for a CoC monster with human-range characteristics (compare to Ghouls and Serpent People), so you might want to tweak it depending on your vision for the Mi Go - those from the old AD&D Deities & Demigods are stupid giant space lobsters that communicate by clicking their nippers.

If you kill one of these Mi Go...

1. It bubbles and rots away in 1 round, leaving a slimy puddle.
2. It abruptly transforms into dense mineral - Dex check at -3 or the killing weapon is stuck. It crumbles to dust in 1-4 rounds.
3. Its flesh blackens and shrivels away, leaving an apparently human skeleton - on the next round, the bones explode for 1-6 hits to all within 10'.
4. It dissolves into a 10' acid pool that evaporates in 1-6 rounds - d8 hits per round and you have to throw away your boots.
5. It bursts into flames, causing 2-8 hits to all within 10'.
6. It transforms into a lifeless duplicate of whoever struck the killing blow, appearing to be a normal dead body in all respects until it disintegrates into black soot after 3 days.

These are Draconian death-throes, from Dragonlance, and I think they suit the weird alien/ 'mysterious Little People' Mi Go just fine.

To these Mi Go, human brains are...

1. Delicious, extracted messily in 1-4 rounds.
2. A delicacy if prepared correctly, extracted carefully over 1-4 turns.
3. Crude computers for their organic robots, shelled like peas in 1-4 rounds.
4. Part of their dastardly plans for infiltration and conquest, carefully removed and sustained with uncanny surgery over 1-4 turns.
5. The raw material for spawning servitors and minions, harvested enthusiastically in 1-4 rounds.
6. Delicate experimental subjects for developing psionic biotechnology, painstakingly and reverentially separated from their host over 1-4 turns.

They definitely want brains, of that I'm sure.

The hummadruz of these Mi Go...

1. Makes humans of 0-1st level drowsy - save vs. spells at +2 per round or nod off; they will have forgotten the 2 minutes prior to nodding off (AD&D 1e forget spell).
2. Hits the Brown Note - save vs. paralysis each round or become nauseated/sick (BECMI Centipede venom, Cave Locust spit, Troglodyte stench etc.) and you have to throw away your underwear.
3. Makes eyes vibrate, ears buzz and noses tickle - save vs. paralysis each round or be afflicted by AD&D 1e blur spell.
4. Fills you with unease and intrusive thoughts - save vs. spells each round or be subject to fear.
5. Prevents spoken, written, telepathic and electronic/mechanical communication.
6. Causes bleeding from every orifice - 1 hit per round, plus save vs. effects as 2 above.

It might require several Mi Go to produce an effective hummadruz - it might even just be a secondary effect of their mass presence in an environment. HPL's The Whisperer in Darkness suggests it can be used by human agents of the Mi Go (presumably through surgical modifications).

In addition to any hand weapons and projectiles, some of these Mi Go are armed with...

1. Energy Mantles: + 5 AC, +5 to all saves; if you strike them, take 2-12 hits plus save vs. paralysis or stunned 1-4 rounds; primarily defensive, but they can make a touch attack vs. unarmoured AC with the same effect; can levitate 12 turns/day, capable of leaving planetary atmosphere/gravity and can sustain one air-breathing prisoner/passenger.
2. Flame Pistols: 1-6 charges, fires a 20' streamer of burning gas for 2-12 hits.
3. Shock Pistols: 1-8 charges, fires a 20' burst of blue sparks for 1-4 hits, but you must also save vs. death (or your heart stops, go to 0 hp) and save vs. paralysis or stunned 1-4 rounds.
4. Cloaking Devices: non-magical illusion to appear as a humanoid up to 10' tall; cannot be disbelieved, can only be seen through with truesight or second sight. Anyone looking at them with dark- or infra-vision will also be fooled, but must save vs. paralysis each round or be affected as hummadruz result 3.
5. Hypnotic Lanterns: flickering blue-white octagonal cylinder mounted on a pole; save vs. spells at +2 each round or fall under non-magical hypnosis - cannot look away from the Lantern and will move as directed by the Mi Go (perfect for brain extractions). Anyone with dark- or infra-vision looking at the Lantern must save vs. paralysis each round or roll 2d3 against the hummadruz results above.
6. Personal Force Fields: protection from normal missiles, resist cold/fire, +3 AC, +1 to all saves, repel anyone within 5' once per round, hurling them 20' for 2-7 hits unless they save vs. dragon breath; can levitate 6 turns/day, but cannot use repel attack while doing so.

Wot no mist projectors or living armour? They're not compulsory.

The purpose of these Mi Go is...

1. To civilise the savages.
2. To domesticate the animals.
3. To uplift the primitives.
4. To extract rare resources.
5. To go boldly/ "What Is This Earth Thing You Call Kissing?"
6. To exterminate all the brutes.

If it isn't clear, by savages, animals, primitives and brutes, I mean you humans. Emergent psychics and/ or spell-casters might be rare resources.

In human terms, these Mi Go are...

1. Absent-minded professors.
2. Evangelists of galactic transhumanity.
3. Unsentimental project managers.
4. Millennially patient and reclusive.
5. Benevolent colonisers heartbroken to punish you for your ingratitude.
6. Full-on Mars Attacks space-bastards.

Even though they seem like the kind of monster that might not, roll Reaction for Mi Go -  though ultimately they aren't really Friendly at all, and their Neutrality can get very chilly, very fast. 

In this case, the Mi Go are fronted by...

1. Scro (Spelljammer space orcs).
2. Mind Flayers and/or Doppelgangers.
3. Oards (BECMI module Where Chaos Reigns).
4. Human agents/cultists (up to and including national governments and 'deep states').
5. Elves.
6. Dragons and/or Githyanki (1st edition AD&D, when they were pals with Red Dragons).

There doesn't need to be a front, and the fronts may use fronts of their own, but Mi Go will always have some indigenous allies, agents or servitors.