NOW UPDATED!!!
This is a list of confirmed monsters in Mordenkainen's Tome of Foes. Please add more as they are announced!
Archdevils: Bael, Geyron, Moloch
Devils: Blue Abishai (Red, White, Black, and Green likely)
Demon Lords: Baphomet (Demogorgon, Orcus, Yeenoghu, Zugtomoy, Grat'tz, Jubliex, and Fraz-Ur'blu expected)
Demons Sibriex, Bulezau, Retriever (technically may be a Construct, but Demon-related)
Shadar-Kai: Gloom Weaver, Shadow Dancer, Soul Monger
Gith: Githyanki Supreme Commander, Githzerai Anarch
Eladrin: Winter Eladrin (Autumn, Spring and Summer likely)
Trolls: Dire, Spirit
Yugoloths: Marraenoloth, Canoloth
Elemental Mrymidons: Air, Earth (Water and Fire likely)
Undead: Nightwalker, Boneclaw, Allip, Vampiric Mist (Skull Lord likely based on picture on alternate cover)
Duergar: four types (will insert names later)
Berbelang
Giff (the hippo people from Spelljammer)
Cadaver Collector
Marut
Leviathan
Phoenix
Nagpa
Astral Dreadnought
Bahalanoth
Skulk
Elder Tempest
Frost Salamander
Tortles: Tortle and Tortle Druid
Bronze Scout
Choker
Eidolon
Star Spawn: number and names unknown
"The Wretched", "The Lonely" and "The Lost" - might be Sorrowsworn based on a reference to "Sorrowsworn wretched" I saw; if not, probably Demons)
I will edit this post periodically as I get more info!