| Name | CR | Type | Environment |
| Tangler | 3 | aberration | any (underground) |
| Tapestry Moth | 16 | magical beast | any |
| Tashtari | 3 | magical beast | any forests or marshes (Castrovel) |
| Tashtari, Tashtari Alpha | 7 | magical beast | any forests or marshes (Castrovel) |
| Tattoo Guardian | 11 | construct (magical) | any |
| Tekenki | 17 | construct (magical, technological) | any (Daimalko) |
| Telia | 2 | monstrous humanoid | any (Pygos) |
| Telia, Telia Raconteur | 7 | monstrous humanoid | any (Pygos) |
| Tenebrous Worm | 8 | outsider (extraplanar) | any land (Shadow Plane) |
| Tetrakam | 9 | outsider (extraplanar) | any (the Drift) |
| Thastertoad | 1 | magical beast | any (Akiton) |
| The Seen | 12 | aberration | any |
| The Swarm, Antecursor | 3 | monstrous humanoid | any |
| The Swarm, Bolaji | 10 | monstrous humanoid | any |
| The Swarm, Ceretoram | 9 | monstrous humanoid | any |
| The Swarm, Convecyte | 2 | monstrous humanoid | any |
| The Swarm, Dissolver | 11 | monstrous humanoid | any |
| The Swarm, Dramovire | 8 | monstrous humanoid | any |
| The Swarm, Dredger | 1/2 | monstrous humanoid | any |
| The Swarm, Effigene | 5 | monstrous humanoid | any |
| The Swarm, Escutchide | 5 | monstrous humanoid | any |
| The Swarm, Klendat | 11 | monstrous humanoid | any |
| The Swarm, Molitera | 6 | monstrous humanoid | any |
| The Swarm, Nauphage | 2 | monstrous humanoid | any |
| The Swarm, Pyschovox | 12 | monstrous humanoid | any |
| The Swarm, Rematern | 7 | monstrous humanoid | any |
| The Swarm, Revulsor | 4 | monstrous humanoid | any |
| The Swarm, Swarm Corrovox | 3 | monstrous humanoid | any |
| The Swarm, Swarm Dredger (Troop) | 5 | monstrous humanoid (troop) | any |
| The Swarm, Swarm Mindreaper | 7 | monstrous humanoid | any |
| The Swarm, Swarm Thresher Lord | 10 | monstrous humanoid | any |
| The Swarm, Teracta | 3 | monstrous humanoid | any |
| The Swarm, Vorphoma | 1 | monstrous humanoid | any |
| The Swarm, Xersk | 16 | monstrous humanoid | any |
| Thermatrod | 3 | magical beast (fire) | any warm mountains |
| Thermatrod, Caldera | 10 | magical beast (fire) | any warm mountains |
| Thermatrod | 3 | magical beast (fire) | any warm mountains |
| Thermophilic Ooze | 3 | ooze (fire) | any warm |
| Thoqqua | 2 | outsider (earth, elemental, extraplanar, fire) | any land (Plane of Fire or Plane
of Earth) |
| Thorgothrel | 8 | ooze | any (Gothra) |
| Threshwolf | 5 | animal | warm and temperate forests and mountains |
| Time Dimensional | 14 | outsider (extraplanar) | any (Dimension of Time) |
| Trinir | 4 | humanoid (trinir) | any (Ashypso) |
| Tripod | 13 | construct (technological) | any |
| Troll Polyp | 3 | aberration (giant) | any |
| Troll | 5 | humanoid (giant) | any |
| Troll, Troll, Quantum | 6 | humanoid (giant) | any |
| Troll, Void Troll | 14 | humanoid (giant) | any |
| Tromlin, Bloodseeker Trooper | 1 | humanoid (tromlin) | any |
| Tromlin, Hardshell Defender | 1 | humanoid (tromlin) | any |
| Trox | 2 | monstrous humanoid | any (Nchak) |
| Trox, Trox Defensor | 9 | monstrous humanoid | any (Nchak) |
| Tryziarka, Explorer | 3 | humanoid (tryziarka) | any |
| Tryziarka, Legacy-captain | 12 | humanoid (tryziarka) | any |
| Tumbletooth Swarm | 5 | fey (swarm) | any |
| Twinsoul | 8 | aberration | any vacuum (Material and Ethereal Planes) |
| Tzitzimitl | 19 | undead | any |