- Imp
- Imp (Dungeon Keeper)
- Imperium of Man
- Imulsion
- Inconnu
- Incubus
- Indominus rex
- Indoraptor
- Indrid Cold
- Infected
- Infected (28 Days Later)
- Infected (Left 4 Dead)
- Infected (The Last of Us)
- Infected Adder
- Infected Alligator
- Infected Bat
- Infected Crow
- Infected Leech
- Infected Moth
- Infector
- Ing
- Inglet
- Innsmouth
- Intelligence (Virus)
- Inugami
- Invaders
- Invisible
- Invisible Haunted
- Invisible Man
- Irad
- Iris
- Iron Golem
- Iron Golem (D&D)
- Iron Hans
- Iron Maiden
- Ironhead
- Irregular Mutant
- Isha
- It
- Ithaqua
- Ivan Ooze
- Ivan T-Types
- Ivy
- Ivy+YX
- Ivy Zombie
- Ivysaur
- Izamu, the Armour
- J'avo
- J'ba Fofi
- Jabberwock
- Jabberwock S3
- Jabir Ibn Hayyan
- Jack Baker
- Jack Krauser
- Jack Norman
- Jack O' Goomba
- Jack O’Lantern (The Cabin in the Woods)
- Jack Skellington
- Jackalope
- Jade
- Jafar
- Jagrafess
- Jake the Dog
- James Marcus
- James P. Sullivan
- Jared Nomak
- Jason Voorhees
- Jataaka
- Javier Hidalgo
- Jedah Dohma
- Jeff the Killer
- Jehovah
- Jelzap
- Jem'Hadar
- Jiang-Shi
- Jigglypuff
- Jinmen
- Jockey
- Johnny
- Jokaero
- Jon Talbain
- Jordy Verrill
- Judoon
- Julian Spiders
- Jumbo Shrimp
- Jumping Maneater
- Juno
- Jupiter
- Jyarimon
- Jörmungand
- Jötunn
- KO Kruiser
- Kaajh'Kaalbh
- Kadabra
- Kageo
- Kaiju
- Kaim
- Kakuna
- Kamacuras
- Kamoebas
- Kan-Ra
- Kantus
- Kappa
- Karen Anatos
- Karl with a K
- Karn
- Kasai Rex
- Keeper
- Keeper (The Evil Within)
- Keeper of the Cage
- Kelpie
- Ken
- Kenfis
- Kennedy Report
- Khorne
- Kia
- Kihunter
- Kijuju Incident
- Kilaak
- Killdozer
- Killer Bee
- Killer Bunny
- Killer Clown
- Killer Croc
- Killer Klowns
- Killer Plant
- Kimodo Dragon
- King Boo
- King Boom Boo
- King Bran the Blessed
- King Bulblin
- King Caesar
- King Ghidorah
- King Koil
- King Kong
- King Sphinx
- King Vendrick
- King Worm
- Kingler
- Kinski
- Kintaro
- Kipepeo
- Kitana
- Kitsune
- Klendathu
- Klixen
- Klyntar
- Knifehead
- Koakuma
- Kobold
- Kodokugumon
- Kog'Maw
- Kolca Threeflies
- Kollector
- Kong (Species)
- Kongamato
- Koopa
- Koopa Kid
- Koopa Paratroopa
- Koopa Troopa
- Koopalings
- Koopatrol
- Koopeleon
- Koromon
- Kotal K'etz
- Kotal Kahn
- Krabby
- Kraid
- Kraken
- Krampus
- Krang
- Kronika
- Kroot
- Kroxigor
- Kryll
- Kthanid
- Kuatan
- Kuchisake-onna
- Kumonga
- Kuo-toa
- Kuramon
- Kushnir Day
- Kytinn
- Kyton
- La Cegua
- La Llorona
- La Magra
- La Mona
- La Santa Compaña
- La Sayona
- Labyrinth (Hellraiser)
- Ladon
- Lake George Monster
- Lakitu
- Lamarr
- Lambent
- Lambent Berserker
- Lambent Brumak
- Lambent Drone
- Lambent Leviathan
- Lambent Pandemic
- Lambent Polyp
- Lambent Stalk
- Lambent Wretch
- Lamia
- Lance
- Lance Soldier
- Landwhale
- Langolier
- Lanshiang Bioterror Incident
- Large Body
- Larry (Rampage movie)
- Larry Koopa
- Las Plagas
- Las Plagas (RE films)
- Lashworm
- Lasombra
- Last Giant
- Laura Victoriano
- Lavos
- Leaderaur
- Leafmon
- Leafwing
- Leaper
- Leatherback
- Leech Zombie
- Leechmonger
- Lemmy Koopa
- Lenny
- Leonardo
- Lepotitsa
- Leprechaun
- Leprocaun
- Lernaean Hydra
- Leroy Gomm
- Leto
- Leveticus
- Leviathan
- Leviathan (Dead Space)
- Leviathan (Gears of War)
- Leviathan (Hellraiser)
- Leviathan (Mass Effect)
- Leviathan (Metroid)
- Li Yentsui
- Liandri Grand Tournament
- Libertus
- Librarian
- Lich
- Lich (Adventure Time)
- Licker
- Licker (RE films)
- Licker β
- Lictor
- Lighthammer
- Lightning Master
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