| Name | CR | Type | Environment |
| Ib Shade | 4 | undead (incorporeal) | temperate swamp |
| Ichthyocentaur | 5 | monstrous humanoid (aquatic) | temperate oceans or coastlines |
| Idol, Bone | 2 | construct | any |
| Idol, Jade | 4 | construct | any |
| Idol, Stone | 3 | construct | any |
| Idol, Wood | 1 | construct | any |
| Ifrit | 1/2 | outsider (native) | any land |
| Ijiraq | 9 | fey (shapechanger) | cold forests or plains |
| Ikelos | 7 | outsider (extraplanar, incorporeal) | any |
| Iku-Turso | 8 | aberration (aquatic) | cold or temperate water or underground |
| Immortal Ichor | 17 | ooze (extraplanar) | any ruins or underground |
| Immured | 3 | undead | any urban |
| Impaler Shrike | 3 | animal | any temperate |
| Implacable Stalker | 8 | outsider (chaotic, demon, evil, extraplanar) | any |
| Impundulu | 11 | outsider (evil, extraplanar, shapechanger) | temperate or warm forest |
| Incutilis | 2 | aberration (aquatic) | any oceans |
| Incutilis, Incutilis Lord | 8 | aberration (aquatic) | any oceans |
| Indarugant | 14 | undead | cold mountains |
| Indescribable Swarm, Pallid Wriggler Swarm | 2 | aberration (swarm) | any underground |
| Indescribable Swarm, Patchwork Insect Swarm | 1/2 | vermin (swarm) | any underground |
| Indescribable Swarm, Swimming Eye Swarm | 4 | aberration (aquatic, swarm) | temperate or warm water |
| Inevitable, Arbiter | 2 | outsider (extraplanar, inevitable, lawful) | any |
| Inevitable, Hykariut | 18 | outsider (extraplanar, inevitable, lawful) | any urban |
| Inevitable, Impariut | 10 | outsider (extraplanar, inevitable, lawful) | any urban |
| Inevitable, Kastamut | 6 | outsider (extraplanar, inevitable, lawful) | any |
| Inevitable, Kolyarut | 12 | outsider (extraplanar, inevitable, lawful) | any |
| Inevitable, Lhaksharut | 20 | outsider (extraplanar, inevitable, lawful) | any |
| Inevitable, Marut | 15 | outsider (extraplanar, inevitable, lawful) | any |
| Inevitable, Novenarut | 4 | outsider (extraplanar, inevitable, lawful) | any (Axis) |
| Inevitable, Rokyamut | 19 | outsider (extraplanar, inevitable, lawful) | any (Axis) |
| Inevitable, Valharut | 11 | outsider (extraplanar, inevitable, lawful) | any (Axis) |
| Inevitable, Yarahkut | 14 | outsider (extraplanar, inevitable, lawful) | any |
| Inevitable, Zelekhut | 9 | outsider (extraplanar, inevitable, lawful) | any land (lawful plane) |
| Inkanyamba | 13 | magical beast | tropical rivers and lakes |
| Intellect Devourer | 8 | aberration | any underground |
| Inverted Giant | 11 | humanoid (giant) | any |
| Invisible Stalker | 7 | outsider (air, elemental, extraplanar) | any (Plane of Air) |
| Irii | 19 | outsider (extraplanar, shapechanger) | any (Dimension of Time) |
| Irlgaunt | 13 | aberration | any mountains or underground |
| Iron Cobra | 2 | construct | any |
| Iron Rhinoceros | 11 | construct | warm mountains or underground |
| Irradiated Dead | 7 | undead | any (Numeria) |
| Isitoq | 1/2 | undead | any |
| Isonade | 15 | magical beast (aquatic) | any oceans |
| Ixion Worm | 13 | outsider (evil, extraplanar, lawful) | any (Hell) |