Missed the first page, but oh well.
This an alternate list of MitD contenders to the FBS/Proposed Ideas list that exists in the OP. This list does not contain any creatures past the cut-off date, nor any non-D&D creatures, including those from call of cthulu, pathfinder, or homebrew. It also does not take Creature Size into account, as it has been suggested on multiple occasions that the MitD is smaller than it usually would be.
Spoiler: Vetted Candidates
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Spoiler: Glabrezu
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Has wish 1/month
Can only grant other peoples' wishes and they have to bring suffering to the world.
No Earthquake
31 STR
Ugly with high charisma.
Telepathy rules a little unclear, but doesn't appear to be able to speak common.
Can speak other languages
Demons don't have children and parents.
Is an embodiment of pure evil. (Always Chaotic Evil)
CR weaker than the order, not something Xykon would consider powerful.
Damage Reduction
Two eyes.
Not Really beast like
Spoiler: Hagunemnon(Protean)
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No wish.
Access to greater teleport, but only via complicated borrowing of other creatures' extraordinary abilities
No Earthquake, but does have very high strength.
53 STR
Ugly with high CHA
By default, doesn't have two eyes. Constantly maintaining the same form goes against fluff and stretches believability.
Speak and understand all languages, but it's default language is incomprehensible to non-proteans.
Parents and Children
Mind reading might explain some knowledge, but no foreshadowing of mind-reading ability.
Has DR
Hard to consider beastlike unless it's holding it's form that way.
High CR
Always Chaotic Neutral
Spoiler: White/Black Slaad
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36/42 STR
No wish, but has greater teleport.
No Earthquake
Speaks language, including common.
Can be mind controlled
Ugly with high CHA
In oots-verse at least, has parent/children.
Would have to be very old.
Slaad in general have appeared elsewhere in OOTS, and the eyes don't match up.
Damage Reduction
Beastlike (Monstrous Frog)
High CR
Always/Usually Chaotic Neutral
Spoiler: Uvuudaum
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39 STR
No wish, but has greater teleport.
No Earthquake
Ugly with high CHA
Speaks telepathically.
No eyes or mouth
Spellcraft and various Knowledge ranks.
Confusion aura might tie into circus scene.
Damage Reduction
Has the general shape of a beast, even though its legs are arms.
Parent/Child?
High CR
Usually Neutral Evil
Spoiler: Ha-naga
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27 STR
Wish
Earthquake
Horrifying abberation with beautiful human face.
Not listed as speaking any language.
Lack of arms suggests difficulty pulling.
Knowledge(Arcana/Religion/History) and Spellcraft ranks.
Damage Reduction
Two eyes
Beast-like
High CR
Usually Chaotic Evil
Spoiler: Grey Linnorm
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High CR
Access to Miracle
Earthquake
24 Strength but can launch people 70 feet.
Ugly with semi-high CHA (17)
Fluff says young ones are rarely seen and the species might be dying out.
Has spellcraft and Arcana ranks.
USUALLY chaotic evil.
Dragon evolutionary offshoot, so Beast-like.
Two eyes
Doesn't speak common.
Speaks Infernal and Draconic
Spoiler: Under Evaluation
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Anaxim (Construct, No teleportation, speaks common)
Astral Dreadnaught (Eyes are wrong, no teleportation)
Atropal (Doesn't eat or Sleep)
Barghest(Dimension door is highest possible teleportation)
Braxat (CR 9, only dimension door)
Cerebrilith (SR)
Corpse Tearer Linnorm (MM2 141)
Dao (MoP)
True Dragon (SRD)
Dream Larva (SRD or ELH 161)
Dread Linnorm (MM2 141)
Dream Larva (SRD or ELH 161)
Efreet (SRD)
Epic Dragon (SRD or ELH 181)
Fiendwurm (MM2 99) (Borderline No Escape Explanation)
Formian Myrmarch (SRD)
Genie/Djinn (SRD or MM 114)
Ghour (MoF)
Gibbering Orb (SRD or ELH 191) (Borderline No Escape Explanation)
Gloom (SRD)(Borderline No Escape Explanation)
Grey Slaad (MM 231)
Guardinal, Leonal (SRD)
Half-Giant (SRD or XPH 200)
Hellfire Wyrm (MM2 125)
Hoary Hunter (SRD or ELH 197)
Hunefer(SRD)
(Under Construction)
Spoiler: Monsters with Major Issues
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Spoiler: Monsters with no inherent Escape Scene explanation besides magic items and HD/class levels
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Aboleth Mage (SRD)
Brachyurus (SRD)
Brood Keeper (MM3)
Centaur (SRD)
Chichimec (SRD)
Couatl (SRD)
Disenchanter (FF)
Displacer Beast (MM 60)
Earth Elemental (SRD or MM 97)
Fhoimoren Giant (?)
Fihyr, Greater (MM2)
Gelatinous Cube (SRD)
Geriviar(MM3)
Gray Render (SRD or MM 138)
Hollyphant(BoED)
Illithid (MM)
(Under Construction)
Spoiler: Published too late to be MitD
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Athasian Nightmare Beast (year late, never officially released. Exists only on defunct forums.)
Alhoon (Lords of Madness is too late.)
Angel of Decay (Libris Mortis is too late)
Astral Dragon (Dragon 344 is too late)
Brainstealer Dragon (Dragon 337)
Concordant Killer (MM4)
Ephemeral Hangman (ToM 161)
Essence Reaver (?)
Goristro(FC1)
Grootslang(5e?)
(Under Construction)
Spoiler: Specific D&D/OOTS related entities
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Aurumach Rilmani
Bauhei
Crystalle
Giant Snowman
Girard (Member of the Order of the Scribble)
Hephaestus
(Under Construction)
Spoiler: 3rd edition Source Needed
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Andeloid
Baku
Black Troll
Cherub
Chiyou (Chinese mythical figure, Has D&D stats?)
Coeurl
Draknor
Draeden
Enveloper (1e)
General Ox
Giant Space Hamster
(Under Construction)
In addition, this is a list of all official D&D 3rd edition products that the MitD could be hiding in. Bolded entries have a monster section.
Spoiler: Strip 100 came out Sep 2, 2004, so no creature first published after that time can be considered.
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Player's Handbook: Core Rulebook I July 2, 2000
Dungeon Master's Guide: Core Rulebook II September 1, 2000
Monster Manual: Core Rulebook III October 1, 2000
Monster Manual II September 1, 2002
Player's Handbook: Core Rulebook I v.3.5 July 1, 2003
Dungeon Master's Guide: Core Rulebook II v.3.5 July 1, 2003
Monster Manual: Core Rulebook III v.3.5 July 1, 2003
Monster Manual III April 1, 2003
Arms and Equipment Guide March 1, 2003 Mounts and Transport Creatures. Includes a few CR 20 but none fit.
Book of Challenges: Dungeon Rooms, Puzzles, and Traps June 1, 2002
Book of Exalted Deeds October 1, 2003
Book of Vile Darkness October 1, 2002
Complete Divine May 1, 2004 (Bogun only, doesn't fit.)
Complete Warrior December 1, 2003
Defenders of the Faith May 1, 2001
Deities and Demigods February 1, 2002 Olympian, Egyptian, and Asgardian Monsters only, none fit.
Draconomicon November 1, 2003
Enemies and Allies October 1, 2001 - You'd think so, but no. Just standard monsters with class levels.
Epic Level Handbook July 1, 2002
Expanded Psionics Handbook April 1, 2004
Fiend Folio April 1, 2003
Ghostwalk June 1, 2003 None fit.
Hero Builder's Guidebook December 1, 2000
Manual of the Planes September 1, 2001
Masters of the Wild February 1, 2002 Dire and Legendary animals, none fit.
Miniatures Handbook October 1, 2003
Oriental Adventures October 1, 2001
Planar Handbook July 1, 2004
Psionics Handbook March 1, 2001
Races of Stone August 1, 2004 192 Races only, none fit.
Savage Species February 1, 2003 Small section, most of this is templates and monster classes (as well as prestiege classes for monsters.
Song and Silence December 1, 2001
Stronghold Builder's Guidebook May 1, 2002
Sword and Fist January 1, 2001
Tome and Blood July 1, 2001
Unearthed Arcana February 1, 2004 Races only, none fit.
Faerun:
Faiths and Pantheons May 2002
Forgotten Realms Campaign Setting 2001
Lords of Darkness 2001
Magic of Faerūn 2001 Very few
Monsters of Faerūn 2001
Player's Guide to Faerūn March 2004
Races of Faerūn March 2003 A few.
Serpent Kingdoms July 2004
Silver Marches 2002 Very few
Unapproachable East May 2003
Underdark October 2003
Eberron:
Eberron Campaign Setting June 2004
Dragonlance:
Dragonlance Campaign Settling, August 2003
Bestiary of Krynn, 2004 Revised 2006
Age of Mortals 2003 (Races only)
Towers of High Sorcery 2004
Magazines
Dragon Magazine: 274-322
Dungeon Magazine: 82 to 113
Vote is A -> D-> C -> B -> Z