| Name | CR | Type | Environment |
| Yaddithian | 8 | aberration | any |
| Yah-Thelgaad | 14 | aberration | any |
| Yangethe | 9 | aberration | cold forests or vacuum |
| Yaoguai | 7 | magical beast | any land |
| Yellow Musk Creeper | 2 | plant | temperate or warm forests and underground |
| Yeth Hound | 3 | outsider (evil, extraplanar) | any |
| Yeti | 4 | monstrous humanoid (cold) | cold mountains |
| Yhohm | 4 | outsider (extraplanar, fire, good) | any (Nirvana) |
| Yithian | 9 | aberration | Any |
| Yithian Elder | 13 | aberration | any |
| Yolubilis Heron | 6 | animal | temperate rivers and swamps |
| Ypotryll | 15 | magical beast | temperate marshes, hills, or coasts |
| Yrthak | 9 | magical beast | temperate deserts, hills, or mountains |
| Yuki-Onna | 8 | undead (cold, incorporeal) | any cold |
| Yurei | 12 | undead | any land |
| Yzobu | 1 | animal | cold hills and plains |