| Name | CR | Type | Environment |
| Feeder Fungus | 4 | plant | any temperate and warm land |
| Ferran | 5 | humanoid | any high-gravity (Ratheren) |
| Ferrofluid Ooze, Ferrofluid Magneto-Cluster | 12 | ooze | urban |
| Ferrofluid Ooze | 2 | ooze | any |
| Fire Whale | 16 | magical beast (extraplanar) | any |
| Flayer Leech | 4 | vermin | any land |
| Flayer Leech, Flayer Leech Effigy | 15 | monstrous humanoid | any land |
| Flying Viper Eel | 10 | animal (swarm) | any air (Bretheda) |
| Formian, Formian Myrmarch | 10 | monstrous humanoid | any land or underground (Castrovel) |
| Formian, Formian Queen | 17 | monstrous humanoid | any land or underground (Castrovel) |
| Formian, Formian Taskmaster | 7 | monstrous humanoid | any land or underground (Castrovel) |
| Formian, Formian Warrior | 3 | monstrous humanoid | any land or underground (Castrovel) |
| Formian, Formian Worker | 1/2 | monstrous humanoid | any land or underground (Castrovel) |
| Free Captains, Free Captain | 4 | humanoid (kasatha) | — |
| Free Captains, Pirate Lord | 7 | humanoid (vesk) | — |
| Free Captains, Space Pirate | 1 | humanoid (human) | — |
| Frujai, Frujai Colony | 19 | plant | any forests (Orikolai) |
| Frujai, Frujai Soldier | 12 | plant | any forests (Orikolai) |
| Fuhlgeist | 8 | outsider (incorporeal) | any |
| Fulmivar | 11 | outsider (elemental, extraplanar) | any |