- Iannic-ann-ôd (Breton) – Ghost of a drowned person
- Iara (Brazilian) – Female water spirit
- Ibong Adarna (Philippine) – Bird that changes color when it finishes a song
- Ichchhadhari Nag (Hindu) – Shapeshifting venomous snakes
- Ichimoku-nyūdō (Japanese) – One-eyed kappa from Sado Island
- Ichiren-Bozu (Japanese) – Animated prayer beads
- Ichneumon (Medieval Bestiaries) – Dragon-killing animal
- Ichthyocentaur (Greek) – Human-fish-horse hybrid
- Iele (Romanian) – Female nature spirits
- Ifrit (Arabian) – Fire genie
- Ijiraq (Inuit) – Spirit that kidnaps children
- Ikiryō (Japanese) – Can be considered a 'living ghost', as it is a person's spirit outside their body
- Ikuchi (Japanese) – Sea serpent that travels over boats in an arc while dripping oil
- Iku-Turso (Finnish) – Sea monster
- Il-Belliegħa (Maltese) – Malevolent well spirit
- Imp (Medieval) – Small demonic servant
- Impundulu (Southern Africa) – Avian, vampiric lightning spirit
- Imugi (Korean) – Flightless, dragon-like creatures (sometimes thought of as proto-dragons)
- Inapertwa (Aboriginal) – Simple organisms, used by creator-gods to make everything else
- Incubus (Medieval folklore) – Male night-demon and seducer
- Indrik (Russian) – One-horned horse-bull hybrid
- Indus Worm (Medieval Bestiaries) – Giant, white, carnivorous worm
- Inkanyamba (Zulu) – Horse-headed serpent
- Inugami (Japanese) – Dog spirit
- Ior (Romanian) – Giant creature, with good spirit
- Ipotane (Greek) – Two-legged horse-human hybrid, (as opposed to the four-legged centaur)
- Ippon-datara (Japanese) – One-legged mountain spirit
- Iratxoak (Basque) – Small demonic servants
- Irin (Jewish) – Fallen angels
- Ishigaq (Inuit) – Little people
- Island Satyr (Medieval Bestiaries) – Savage human-goat hybrid from a remote island chain
- Isonade (Japanese) – Shark-like sea monster
- Ittan-momen (Japanese) – Ghostly aerial phenomenon that attacks people
- Iwana-bōzu (Japanese) – Char which appeared as a Buddhist monk