7 of 7 minor update

JavaScript (ES6), 915 ... 902 890 bytes

w=>[..."aZM?o@;LWu&P?D@zF@W: @aT&@nCEfvQ&R&Tb'b@&p@:Srn @ahlrdpdT'TRv:HUYG@&fSfYdG&SGHL@Mh@G@gs';@CS@km@OsirA@q@njOZS@O@';HYqHE&DJavq&&aYaBmZMf;bv@EqHg@Z@;dm@M@?@rs@d@@oDAosDT@d@ZeBVrq@jFooD@VV&&BvMEDKiuiPC@&@DYrD&eD@D@@:AwccKZiF:DKLXAwdL@w&@@u'Hc@@q&;D:::WjdN@N@xD&eFh@gh@&Md?&Ye@@&h@hNN'Z&qtKEd@@HtH&@'@&@xd&dZsv@oo@FDyd@@&&@&@HS'Hw?DF@@@MPfDfi'AH&@@pkZkuMyZhFNN'P?d@u@NN&B@uo'fdi@?ke&"].find((_,i)=>!(s-=`GD4~#_@'R<1*~7C7RbZ6F'"Sa&!*1),#''3'.+B6(K$.l%9&!#0@51""~/+!gaW!/.(5'-@0'';!%C.&""!-.$16.2>(#&g!!O,#8A50O!)*(9b|Z4@7V).;*A*HWO(g1$/*-4&SL1I#K$#"3"#=e/'V~4'B(*,.3),$@D3)*76-"\\&kL7(-4#=7!!#+(B/B!-%!"_+!")+)0$1:E84!L191&)(255)!3O<,90NN6&;Q2'"bO=*h7.%1![<o!%M'G5/R.0$-J*%\\~6T?>)16""L&!X94T4"3$!2'^070Y2a"##)#"&n&(+1*&!-M""73R5%'y0~$-6<".MV?+1*ED>!B6b!)%&)8.+$&X0~Q'E%8&#%S/H.1<#>~!sU`.charCodeAt(i)-32),w=w.replace(/\W/g,1),s=parseInt((w+=w+w)[0]+w[2]+w[3]+w[6]+[...w].pop(),36)%8713)

Formatted

Below is a formatted version of the code with truncated payload data.

w => [..."aZM(…)"].find(
  (_, i) =>
    !(s -= `GD4(…)`.charCodeAt(i) - 32),
    w = w.replace(/\W/g, 1),
    s = parseInt((w += w + w)[0] + w[2] + w[3] + w[6] + [...w].pop(), 36) % 8713
)

How it works

Step #1

We first reduce the monster name by:

  1. Replacing non-alphanumerical characters (spaces and dashes) with 1's.
  2. Repeating this string 3 times to make sure that we have enough characters to work with for the next step.
  3. Keeping only the 1st, 3rd, 4th, 7th and last characters.

Examples:

1.34..7..L
Demogorgon -> Dmorn
^ ^^  ^  ^

             1.34..7.L
orc mummy -> orc1mummy -> oc1my
             ^ ^^  ^ ^

        1.34..7....L
xorn -> xornxornxorn -> xrnrn
        ^ ^^  ^    ^

This leads to a few collisions. For instance, "Master Assassin" and "Master Kaen" are both reduced to "Mst1n". Fortunately, all colliding monster names share the same symbol (@ in this case).

Step #2

Then, we interpret this 5-character string as a base 36 quantity to convert it to decimal (this operation is case insensitive) and we apply a modulo 8713, which was empirically chosen to produce a collision-free list of keys.

Examples:

Dmorn --[from base 36]--> 22893539 --[MOD 8713]--> 4488
oc1my --[from base 36]--> 40872778 --[MOD 8713]--> 95
xrnrn --[from base 36]--> 56717843 --[MOD 8713]--> 4926

Step #3

All keys are sorted in ascending order:

[ 39, 75, 95, 192, 255, 287, 294, 344, 372, 389, 399, 516, 551, 574, 624, ..., 8635, 8688 ]

Converted to delta values:

[ 39, 36, 20, 97, 63, 32, 7, 50, 28, 17, 10, 117, 35, 23, 50, ..., 83, 53 ]

And encoded as ASCII characters in the range [ 32, 126 ]. Some intermediate dummy values are inserted when the difference between two consecutive keys exceeds the maximum encodable magnitude.

Finally, the list of keys is mapped to a list of symbols arranged in the same order.

Test

f=
w=>[..."aZM?o@;LWu&P?D@zF@W: @aT&@nCEfvQ&R&Tb'b@&p@:Srn @ahlrdpdT'TRv:HUYG@&fSfYdG&SGHL@Mh@G@gs';@CS@km@OsirA@q@njOZS@O@';HYqHE&DJavq&&aYaBmZMf;bv@EqHg@Z@;dm@M@?@rs@d@@oDAosDT@d@ZeBVrq@jFooD@VV&&BvMEDKiuiPC@&@DYrD&eD@D@@:AwccKZiF:DKLXAwdL@w&@@u'Hc@@q&;D:::WjdN@N@xD&eFh@gh@&Md?&Ye@@&h@hNN'Z&qtKEd@@HtH&@'@&@xd&dZsv@oo@FDyd@@&&@&@HS'Hw?DF@@@MPfDfi'AH&@@pkZkuMyZhFNN'P?d@u@NN&B@uo'fdi@?ke&"].find((_,i)=>!(s-=`GD4~#_@'R<1*~7C7RbZ6F'"Sa&!*1),#''3'.+B6(K$.l%9&!#0@51""~/+!gaW!/.(5'-@0'';!%C.&""!-.$16.2>(#&g!!O,#8A50O!)*(9b|Z4@7V).;*A*HWO(g1$/*-4&SL1I#K$#"3"#=e/'V~4'B(*,.3),$@D3)*76-"\\&kL7(-4#=7!!#+(B/B!-%!"_+!")+)0$1:E84!L191&)(255)!3O<,90NN6&;Q2'"bO=*h7.%1![<o!%M'G5/R.0$-J*%\\~6T?>)16""L&!X94T4"3$!2'^070Y2a"##)#"&n&(+1*&!-M""73R5%'y0~$-6<".MV?+1*ED>!B6b!)%&)8.+$&X0~Q'E%8&#%S/H.1<#>~!sU`.charCodeAt(i)-32),w=w.replace(/\W/g,1),s=parseInt((w+=w+w)[0]+w[2]+w[3]+w[6]+[...w].pop(),36)%8713)

list = {
  "Aleax": "A",
  "Angel": "A",
  "Arch Priest": "@",
  "Archon": "A",
  "Ashikaga Takauji": "@",
  "Asmodeus": "&",
  "Baalzebub": "&",
  "Chromatic Dragon": "D",
  "Croesus": "@",
  "Cyclops": "H",
  "Dark One": "@",
  "Death": "&",
  "Demogorgon": "&",
  "Dispater": "&",
  "Elvenking": "@",
  "Famine": "&",
  "Geryon": "&",
  "Grand Master": "@",
  "Green-elf": "@",
  "Grey-elf": "@",
  "Hippocrates": "@",
  "Ixoth": "D",
  "Juiblex": "&",
  "Keystone Kop": "K",
  "King Arthur": "@",
  "Kop Kaptain": "K",
  "Kop Lieutenant": "K",
  "Kop Sergeant": "K",
  "Lord Carnarvon": "@",
  "Lord Sato": "@",
  "Lord Surtur": "H",
  "Master Assassin": "@",
  "Master Kaen": "@",
  "Master of Thieves": "@",
  "Medusa": "@",
  "Minion of Huhetotl": "&",
  "Mordor orc": "o",
  "Nalzok": "&",
  "Nazgul": "W",
  "Neferet the Green": "@",
  "Norn": "@",
  "Olog-hai": "T",
  "Oracle": "@",
  "Orcus": "&",
  "Orion": "@",
  "Pelias": "@",
  "Pestilence": "&",
  "Scorpius": "s",
  "Shaman Karnov": "@",
  "Thoth Amon": "@",
  "Twoflower": "@",
  "Uruk-hai": "o",
  "Vlad the Impaler": "V",
  "Wizard of Yendor": "@",
  "Woodland-elf": "@",
  "Yeenoghu": "&",
  "abbot": "@",
  "acid blob": "b",
  "acolyte": "@",
  "air elemental": "E",
  "aligned priest": "@",
  "ape": "Y",
  "apprentice": "@",
  "arch-lich": "L",
  "archeologist": "@",
  "attendant": "@",
  "baby black dragon": "D",
  "baby blue dragon": "D",
  "baby crocodile": ":",
  "baby gray dragon": "D",
  "baby green dragon": "D",
  "baby long worm": "w",
  "baby orange dragon": "D",
  "baby purple worm": "w",
  "baby red dragon": "D",
  "baby silver dragon": "D",
  "baby white dragon": "D",
  "baby yellow dragon": "D",
  "balrog": "&",
  "baluchitherium": "q",
  "barbarian": "@",
  "barbed devil": "&",
  "barrow wight": "W",
  "bat": "B",
  "black dragon": "D",
  "black light": "y",
  "black naga hatchling": "N",
  "black naga": "N",
  "black pudding": "P",
  "black unicorn": "u",
  "blue dragon": "D",
  "blue jelly": "j",
  "bone devil": "&",
  "brown mold": "F",
  "brown pudding": "P",
  "bugbear": "h",
  "captain": "@",
  "carnivorous ape": "Y",
  "cave spider": "s",
  "caveman": "@",
  "cavewoman": "@",
  "centipede": "s",
  "chameleon": ":",
  "chickatrice": "c",
  "chieftain": "@",
  "clay golem": "'",
  "cobra": "S",
  "cockatrice": "c",
  "couatl": "A",
  "coyote": "d",
  "crocodile": ":",
  "demilich": "L",
  "dingo": "d",
  "disenchanter": "R",
  "djinni": "&",
  "dog": "d",
  "doppelganger": "@",
  "dust vortex": "v",
  "dwarf king": "h",
  "dwarf lord": "h",
  "dwarf mummy": "M",
  "dwarf zombie": "Z",
  "dwarf": "h",
  "earth elemental": "E",
  "electric eel": ";",
  "elf mummy": "M",
  "elf zombie": "Z",
  "elf": "@",
  "elf-lord": "@",
  "energy vortex": "v",
  "erinys": "&",
  "ettin mummy": "M",
  "ettin zombie": "Z",
  "ettin": "H",
  "fire ant": "a",
  "fire elemental": "E",
  "fire giant": "H",
  "fire vortex": "v",
  "flaming sphere": "e",
  "flesh golem": "'",
  "floating eye": "e",
  "fog cloud": "v",
  "forest centaur": "C",
  "fox": "d",
  "freezing sphere": "e",
  "frost giant": "H",
  "gargoyle": "g",
  "garter snake": "S",
  "gas spore": "e",
  "gecko": ":",
  "gelatinous cube": "b",
  "ghost": " ",
  "ghoul": "Z",
  "giant ant": "a",
  "giant bat": "B",
  "giant beetle": "a",
  "giant eel": ";",
  "giant mimic": "m",
  "giant mummy": "M",
  "giant rat": "r",
  "giant spider": "s",
  "giant zombie": "Z",
  "giant": "H",
  "glass golem": "'",
  "glass piercer": "p",
  "gnome king": "G",
  "gnome lord": "G",
  "gnome mummy": "M",
  "gnome zombie": "Z",
  "gnome": "G",
  "gnomish wizard": "G",
  "goblin": "o",
  "gold golem": "'",
  "golden naga hatchling": "N",
  "golden naga": "N",
  "gray dragon": "D",
  "gray ooze": "P",
  "gray unicorn": "u",
  "green dragon": "D",
  "green mold": "F",
  "green slime": "P",
  "gremlin": "g",
  "grid bug": "x",
  "guard": "@",
  "guardian naga hatchling": "N",
  "guardian naga": "N",
  "guide": "@",
  "healer": "@",
  "hell hound pup": "d",
  "hell hound": "d",
  "hezrou": "&",
  "high priest": "@",
  "hill giant": "H",
  "hill orc": "o",
  "hobbit": "h",
  "hobgoblin": "o",
  "homunculus": "i",
  "horned devil": "&",
  "horse": "u",
  "housecat": "f",
  "human mummy": "M",
  "human zombie": "Z",
  "human": "@",
  "hunter": "@",
  "ice devil": "&",
  "ice troll": "T",
  "ice vortex": "v",
  "iguana": ":",
  "imp": "i",
  "incubus": "&",
  "iron golem": "'",
  "iron piercer": "p",
  "jabberwock": "J",
  "jackal": "d",
  "jaguar": "f",
  "jellyfish": ";",
  "ki-rin": "A",
  "killer bee": "a",
  "kitten": "f",
  "knight": "@",
  "kobold lord": "k",
  "kobold mummy": "M",
  "kobold shaman": "k",
  "kobold zombie": "Z",
  "kobold": "k",
  "kraken": ";",
  "large cat": "f",
  "large dog": "d",
  "large kobold": "k",
  "large mimic": "m",
  "leather golem": "'",
  "lemure": "i",
  "leocrotta": "q",
  "leprechaun": "l",
  "lich": "L",
  "lichen": "F",
  "lieutenant": "@",
  "little dog": "d",
  "lizard": ":",
  "long worm": "w",
  "lurker above": "t",
  "lynx": "f",
  "mail daemon": "&",
  "manes": "i",
  "marilith": "&",
  "master lich": "L",
  "master mind flayer": "h",
  "mastodon": "q",
  "mind flayer": "h",
  "minotaur": "H",
  "monk": "@",
  "monkey": "Y",
  "mountain centaur": "C",
  "mountain nymph": "n",
  "mumak": "q",
  "nalfeshnee": "&",
  "neanderthal": "@",
  "newt": ":",
  "ninja": "@",
  "nurse": "@",
  "ochre jelly": "j",
  "ogre king": "O",
  "ogre lord": "O",
  "ogre": "O",
  "orange dragon": "D",
  "orc mummy": "M",
  "orc shaman": "o",
  "orc zombie": "Z",
  "orc": "o",
  "orc-captain": "o",
  "owlbear": "Y",
  "page": "@",
  "panther": "f",
  "paper golem": "'",
  "piranha": ";",
  "pit fiend": "&",
  "pit viper": "S",
  "plains centaur": "C",
  "pony": "u",
  "priest": "@",
  "priestess": "@",
  "prisoner": "@",
  "purple worm": "w",
  "pyrolisk": "c",
  "python": "S",
  "quantum mechanic": "Q",
  "quasit": "i",
  "queen bee": "a",
  "quivering blob": "b",
  "rabid rat": "r",
  "ranger": "@",
  "raven": "B",
  "red dragon": "D",
  "red mold": "F",
  "red naga hatchling": "N",
  "red naga": "N",
  "rock mole": "r",
  "rock piercer": "p",
  "rock troll": "T",
  "rogue": "@",
  "rope golem": "'",
  "roshi": "@",
  "rothe": "q",
  "rust monster": "R",
  "salamander": ":",
  "samurai": "@",
  "sandestin": "&",
  "sasquatch": "Y",
  "scorpion": "s",
  "sergeant": "@",
  "sewer rat": "r",
  "shade": " ",
  "shark": ";",
  "shocking sphere": "e",
  "shopkeeper": "@",
  "shrieker": "F",
  "silver dragon": "D",
  "skeleton": "Z",
  "small mimic": "m",
  "snake": "S",
  "soldier ant": "a",
  "soldier": "@",
  "spotted jelly": "j",
  "stalker": "E",
  "steam vortex": "v",
  "stone giant": "H",
  "stone golem": "'",
  "storm giant": "H",
  "straw golem": "'",
  "student": "@",
  "succubus": "&",
  "tengu": "i",
  "thug": "@",
  "tiger": "f",
  "titan": "H",
  "titanothere": "q",
  "tourist": "@",
  "trapper": "t",
  "troll": "T",
  "umber hulk": "U",
  "valkyrie": "@",
  "vampire bat": "B",
  "vampire lord": "V",
  "vampire": "V",
  "violet fungus": "F",
  "vrock": "&",
  "warg": "d",
  "warhorse": "u",
  "warrior": "@",
  "watch captain": "@",
  "watchman": "@",
  "water demon": "&",
  "water elemental": "E",
  "water moccasin": "S",
  "water nymph": "n",
  "water troll": "T",
  "werejackal": "d",
  "wererat": "r",
  "werewolf": "d",
  "white dragon": "D",
  "white unicorn": "u",
  "winged gargoyle": "g",
  "winter wolf cub": "d",
  "winter wolf": "d",
  "wizard": "@",
  "wolf": "d",
  "wood golem": "'",
  "wood nymph": "n",
  "woodchuck": "r",
  "wraith": "W",
  "wumpus": "q",
  "xan": "x",
  "xorn": "X",
  "yellow dragon": "D",
  "yellow light": "y",
  "yellow mold": "F",
  "yeti": "Y",
  "zruty": "z"
};

success = error = 0;

for(w in list) {
  list[w] == f(w) ? success++ : error++;
}
console.log("Success:", success);
console.log("Error:", error);