Now that a few long-standing mysteries have been resolved, here is my definitive list of all monsters in BT1, arranged by the picture used to display them.
Mercenary
Mercenary
Swordsman
Bladesman
Warrior Elite
Maze Master
Barbarian
Nomad
Barbarian (in later versions)
Berserker
Bone Crusher
Samurai
Samurai
Samurai Lord
Mangar Guard ("Mandar Guard" in older versions)
Thief / Ninja
Thief
Assassin
Master Thief
Ninja
Master Ninja
Monk
Jade Monk
Scarlet Monk
Azure Monk
Ivory Monk
Conjuror / Magician (same as the character picture of the conjuror)
Conjuror (6 conjurors and magicians, presumably casting spells level 1-6)
Magician
Master Conjurer
Master Magician
Archmage
Sorcerer / Wizard (same as the character picture of the magician)
Sorcerer (5 sorcerers and wizards, presumably casting spells level 1-5 or 2-6)
Wizard
Master Sorcerer
Master Wizard
Old Man
Fred
Mongo
Old Man
Balrog (in earlier versions)
Dwarf
Hobbit
Dwarf
Dwarf King
Goblin
Kobold
Gnome
Goblin Lord
Orc
Hobgoblin
Orc
Half Orc
Ogre
Barbarian (in earlier versions)
Ogre
Ogre Magician ("Ogre Magi" in earlier versions)
Ogre Lord
Giant
Stone Giant
Fire Giant
Ice Giant
War Giant
Titan
Cloud Giant
Storm Giant
Golem
Statue
Stone Elemental
Xorn
Golem
Crystal Golem
Mad God
Mad God
Spider
Spider
Black Widow
Spinner
Animal
Mad Dog
Wolf
Werewolf
Jackalwere
Weretiger
Bandersnatch
Eye
Seeker
Eye Spy
Evil Eye
Beholder
Soul Sucker
Hydra
Hydra
Basilisk
Jabberwock
Dragon
Blue Dragon
Green Dragon
Copper Dragon
White Dragon
Grey Dragon
Red Dragon
Black Dragon
Skeleton
Skeleton
Zombie
Zombie
Doppleganger
Body Snatcher
Mimic
Gimp
Wight / Ghoul
Wight
Ghoul
Spectre
Spectre
Lich (no opportunity to meet these in later versions)
Vampire
Vampire
Vampire Lord
Ghost
Lurker
Ghost
Maze Dweller
Mind Shadow
Death Denizen (no opportunity to meet these in any versions as far as we know)
Demon
Wraith
Shadow
Phantom
Lesser Demon
Demon
Greater Demon
Ancient Enemy
Balrog
Balrog (in later versions)
Demon Lord
Demon Lord (in earlier versions, only met if summoned by an enemy)
Mangar
Mangar
