| Name | CR | Type | Environment |
| Xacarba | 15 | outsider (chaotic, evil, extraplanar) | any land (Abyss) |
| Xanthos | 14 | magical beast | any land |
| Xenopterid | 7 | vermin | any land or underground |
| Xiao | 2 | magical beast | warm mountains or forests |
| Xill | 6 | outsider (evil, extraplanar) | any (Ethereal Plane) |
| Xill, Xill Matriarch | 9 | outsider (evil, extraplanar) | any (Ethereal Plane) |
| Xiomorn, Vault Keeper | 14 | outsider (earth, elemental, extraplanar) | any |
| Xocothian | 4 | outsider (air, extraplanar, water) | any (Plane of Air or Plane of Water) |
| Xorn | 6 | outsider (earth, extraplanar) | any (Plane of Earth) |
| Xtabay | 1/2 | plant | any land |
| Xulgath | 4 | humanoid (reptilian) | any underground (Deep Tolguth) |