- Gaasyendietha (Seneca) – Dragon
- Gagana (Russian) – Iron-beaked bird with copper talons
- Gaki (Japanese) – Ghosts of especially greedy people
- Gaokerena (Zoroastrianism) - Mythic plant
- Gallu (Mesopotamian) – Underworld demons
- Galtzagorriak (Basque) – Small demonic servants
- Gamayun (Russian) – Prophetic human-headed bird
- Gana (Hindu) – Attendants of Shiva
- Gancanagh (Irish) – Male fairy that seduces human women
- Gandabherunda (Hindu) – Double-headed bird
- Gandharva (Hindu) – Male nature spirits, often depicted as part human, part animal
- Gargouille (French) – Water dragon
- Garkain (Australian Aboriginal) – Flying humanoid who envelops his victims
- Garmr (Norse) – Giant, ravenous hound
- Garuda (Hindu) – Human-eagle hybrid
- Gashadokuro (Japanese) – Giant malevolent skeletons
- Gaueko (Basque) – Wolf capable of walking upright
- Geb (Egyptian) – God of the Earth, married to Nut
- Ged (Heraldic) – The fish pike
- Gegenees (Greek) – Six-armed giant
- Genius loci (Roman) – Spirit that protects a specific place
- German (Slavic) – Male spirit associated with bringing rain and hail
- Geryon (Greek) – Three-headed six-armed giant with three torsos and (in some sources) six legs
- Ghillie Dhu (Scottish) – Tree guardian
- Ghost – Disembodied spirits of those that have died
- Ghoul (Arabian) – Cannibalistic shapeshifting desert genie often classified as undead.
- Giant (Worldwide) – Immensely large and strong humanoids
- Giant animal (Worldwide) – Unusually large beasts
- Gichi-anami'e-bizhiw (Ojibwa) – Bison-snake-bird-cougar hybrid water spirit
- Gidim (Sumerian) – Ghost
- Gigantes (Greek) – Race of giants that fought the Olympian gods, sometimes depicted with snake-legs
- Gigelorum (Scottish) – Smallest animal
- Girtablilu (Akkadian) – Human-scorpion hybrid
- Gjenganger (Scandinavian) – Corporeal ghost
- Glaistig (Scottish) – Human-goat hybrid
- Glashtyn (Manx) – Malevolent water horse
- Gnome (Alchemy) – Diminutive Earth elemental
- Goblin (Medieval) – Grotesque, mischievous little people
- Gog (English) – Giant protector of London
- Gold-digging ant (Medieval Bestiaries) – Dog-sized ant that digs for gold in sandy areas
- Golem (Jewish) – Animated construct
- Gorgades (Medieval Bestiary) – Hairy humanoid
- Gorgon (Greek) – Fanged, snake-haired humanoids that turn anyone who sees them into stone
- Goryō (Japanese) – Vengeful ghosts, usually of martyrs
- Grassman (Ohio, USA) – Ape-like cryptid
- Gremlin (Folklore) – Creatures that sabotage airplanes
- Griffin (Heraldic) – Lion-eagle hybrid
- Grigori (Christian, Jewish, and Islamic mythology) – Fallen angels, father of Nephilim
- Grim (English and Scandinavian) – Tutelary spirits of churches
- Grim Reaper (Worldwide) – Death angel often thought to be God's/Satan's assistant
- Grindylow (English) – Malevolent water spirit
- Gualichu (Mapuche) – Malevolent spirit
- Guardian angel (Christian, Jewish, and Islamic belief) – Subclassification of angels that guard and protect a specific person or living being
- Gud-elim (Akkadian) – Human-bull hybrid
- Guhin (Japanese) – Anthropomorphic bird
- Gui Po (Chinese) – Ghost that manifests as an old woman
- Gui Shu (Chinese) – Ghostly tree that confuses travelers by moving
- Gulon (Germanic) – Gluttonous dog-cat-fox hybrid
- Gumiho (Korean mythology) – Demonic fox with thousands of tails believed to possess an army of spirits and magic in its tails
- Gurangatch (Australian Aboriginal) - An enormous reptile-fish whose movements carved out the landscape south of the Blue Mountains
- Gurumapa (Nepalese) – Child-eating demon
- Gwyllgi (Welsh) – Black dog
- Gwyllion (Welsh) – Malevolent spirit
- Gyascutus (American folklore) – Four-legged herbivore
- Gytrash (Lincolnshire and Yorkshire) – Black dog
- Gyūki (Japanese) – Bull-headed monster