d100 How did these guys got into this dungeon room

OK whose asked foolish questions like

How did those 20 orcs get in the room at end of a 100 foot yellow mould corridor?
How did this ettin get in this room with a goblin door?
How did the goblins get a killer whale in this indoor cave pool?
How did walrus adapt to living in the forest? (not relevant sorry)

So now you can just laugh people away. Funhouse dungeons are basically mad mage murder mazes so can have every room an impossible wonder to confuse you. They can even be dreamlike or have illusionary properties. Most dungeons are a bit more sensible and often organised outposts based on rational military and wizard and evil cult principles, not just random chaos tainted wild magic places. So to help explain stuff try these. You can leave clues like a book of expositions or something more subtle like remains of history.

If your sensible dungeon would not be able to do all these things but you could have a petty god came through or they invited a jerk wizard to make traps and they did weird shit we don't get. I do like the DnD5 mad mage has his face and simulacra of himself all over the dungeon. Have not found lots of love for this book.

If the monsters have problems getting out, consider they might be hungry or have a food macking item or were good preppers with boxes of food or use a priest spell to make food. Perhaps they live off giant rats and suck lichen for juice. Possibly being in here is a trap or a punishment,

Go on make a dungeon that's really a prison for tortured monsters you get to save by killing and getting past custom traps vs monsters

So when you see the how did this happen thread in a dungeon design involving how some monster got in some implausible place hit them with this d100.

d12 Signs The Monster Is Supposed To Be Here Long Term
1 Remain of other dead creatures eaten or starved in here

2 Chained to walls unable to leave
3 Abusive text on the wall, a personal attack on the monster
4 Monsters features are worked into artwork
5 Room is a purpose-built habitat or shelter with crates of food
6 Magic food and water bowls fill once per day
7 Creature was in stasis until intruders ever
8 Creature is sleeping or hibernating in a cacoon or hidden nest
9 Signs someone has been feeding through a secret hatch in ceiling or wall 
10 Luxurious baroque lair and shiny treasure, the monster has good grooming, attended by spirits of unseen servants who bring the beast cake and wine and fancy feasts
11 Prisoner in here creates food and water with spells for monster
12 Food and water for monster dropped through pipes or a trapdoor 

d100 How did these guys got into this dungeon room
01 Monster was brought in here as a baby and was fed as an adult, keepers told it outside is a deadly poison
02 Monster as baby was raised by another monster mom who dragged it here
03 Monster was brought in as a baby then cult called a ghost to age it
04 Monster was brought in small and was force grown with a potion or spell
05 Monster was raised since baby by unseen servants and food and water arrives by magic daily  
06 Monster was brought in by other monsters who lived here a time then moved on
07 Wizard created monsters in this room, shattered apparatus and waste is in piles in room
08 Wizard summoned creatures here and abandoned
09 Monsters wandered in and living of some strange magic giant beehive and royal jelly
10 Monster was teleported away at random by a wizard from home
11 God or powerful being punished monster by placing here
12 Monster placed in a room by wizard and room is stasis until door just opened
!3 A chaos jester petty god came to visit and did this as a joke
14 Was sealed in here alive for a long time as punishment
15 Trapped in here during some construction mistake
16 Were meddling with a cursed relic (possibly now with them) 
17 Every day a dungeon spirit appears and makes food and beer for inhabitants
18 An imp arrives to feed inhabitants trapped in here to make them sell souls
19 An unstable gateway shunts stuff here and strange junk is in the room too
20 Creatures trapped inside have a troll chained up that they eat to survive
21 The room is under some natural temporal spacial field 
22 A deity put this here as a lesson to its cult and the inhabitants
23 Game in through a hole while shrunk by a potion 
24 Came in with a gaseous form potion and trapped
25 Came in polymorphed and then magic dispelled 
26 Crypt Keeper teleported them in here 
27 Was brought in unconscious and sealed in by some crazy boss
28 Was lost in a dimensional void and slipped back to reality here
29 Area is haunted and shifts into otherworld with different passages elswhere
30 Area was visited by faeries who left some monsters here who became boring
31 In the room is pile of rubble where burrowed in and was trapped
32 Boss hired a wizard to deliver monster and it just wound up here no refunds
33 The creature was starving and runty when squeezes in here since has recovered and grown
34 Demon put the monster here as a reward for a minion and monsters here since
35 Monsters were frozen and walls modified during suspended animation
36 Carried here in a portable hole and it got out and hole owners fled long ago
37 Polymorphed or great cursed adventurer turned into a monster
38 Devil brought polymorphed victim here as a doublecross wishing for something stupid 
39 Monster brought here by wizard awaiting hostage payment, monster was human
40 Monster is a statue becomes flesh when intruders enter
41 Monsters brought in as a pet when a baby then neglected, eventually it laired in here feeding off giant rats till to fat to leave
42 Monster sealed in here as punishment by crafters but fed occasionally
43 Shattered remains of a magical urn the creature was brought in and grew from
44 Escaped from a miniature world hidden in the cracks under the floor 
45 Summoned by a dungeon spirit to protect spirits totem object
46 Adventurer malfunctioned a magic item and then smashed it
47 Adventurer triggered a magical trap turning them into monmster/s
48 Monster transmuted into figurine but entered an antimagic shell 
49 Monster drank a gaseous form potion
50 Brought here and changed and imprisoned by hags curse
51 Angel imprisoned monster here who gets enough manna to live but always hungry
52 Fell in through lost upper level and trapped
53 Crawled out of drain, grew up here
54 Broken alchemist apparatus made monster
55 Broken gate portal or teleporter pad brought monster who broke aparatus
56 Remains of broken monster making machine from hell
57 Monster formed from slime, alchemical waste and duetritis 
58 Small monster ate a weird mushroom and turned into bigger one
59 Small animal accidentally exposed to raw chaos from beyond
60 Cult used magic to imprison occupants to guard an occult treasure
61 Monster is really a fungus clone grown here from spores
62 Clone grown in here by a wizard, remaining apparatus parts remain and dead wizard
63 Monster used to be a wizard who made a mistake
64 Monster betrayed cult and imprisoned as a monster
65 Monster in stasis for aeons and room built to incorporate like a statue, then freed by a cultist
66 Betrayed a cult and made into a monster using torture and potions
67 Someone cast rock to flesh on a fossil petrified monster in-wall and it ate them
68 Brought here by an angel to punish it
69 Humanoids wonderous child grew into this thing, the mother won't say who the father is
70 Hatched from egg brought here by monsters to store 
71 Priest assembled creatures remains from small bits then brought back to life here
72 Wizard experimenting with pets and failed potions
73 Grew from magic beans
74 Came in a magic chest sent by evil wizard or demon
75 Necromancy students assembling a corpse
76 Strange plant grew pods that hatched into creature a plant clone
77 Formless shapeshifting creature of flesh was cursed and locked in this form 
78 Actually fused with troll flesh and a clone regrown from a body part here
79 Monster drank from a magic fountain and shrink, later becoming trapped here 
80 Broke in here and others repaired damage trapping it
81 Was trapped in a magic painting but escaped, broken painting remains
82 Was trapped in a magic mirror but escaped, broken frame remains
83 Was trapped in a magic crystal but escaped, broken crystal shards remain
84 A priest poured out long-sealed blood of a demon lord and the monster grew from it
85 A Magic horn summoned the thing, its broken remains are scattered 
86 A wall hides an unstable phase shifted demiplane the thing came from
87 Dead goblin with shattered prismatic goggles that drew the thing into this world
88 Someone broke a cursed tablet freeing the monster from the netherworld
89 Priest brought in as dust in urn and resurrected
90 Cursed weapon released simulacra of last thing slain
91 Trickster god put here to reinforce faith in miracles among murder hobos
92 An orc god places here to kill adventurers who have harmed orcs ever
93 A spirit can manifest as a monster as a temporary physical form
94 Trapped in a magic carpet then released here
95 Some great demon was called here bringing monsters pets and leaving this behind
96 A person is cursed and becomes a monster if they try to leave this luxury cell
97 Tiny monster when sees intruders will eat mushroom to return to size for ambush
98 An ancient magic container was opened releasing imprisoned monster
99 Monster reintegrated from dust by a time field to life
100 Summoned then abandoned by nighthags

if any too similar let me know