Hydra

The Monsters Know What They're Doing     Dungeons And Dragons original/official source

Atlas Animalia     Any original/official source

OSE Encounter Activity Tables     Any original/official source

Pathfinder 1e Bestiary 1     Pathfinder original/official source

Pathfinder 2e Bestiary 1     Pathfinder original/official source

Mythical Monsters Revisted     Pathfinder (no source link)

Mythic Adventures     Pathfinder original/official source

Monster Manual     D&D 5e original/official source

Monster Monday (web)     D&D 5e original/official source

Monster Ecology Anthology     D&D 5e (no source link)

Monster Manual     D&D 4e (no source link)

Monster Manual 2     D&D 4e (no source link)

Dungeons And Dragons Essentials: Monster Vault     D&D 4e (no source link)

Dragon Magazine     D&D 2e/3e (no source link)

Scruffy Grognard's Monstrous Manual     D&D 2e (no source link)

Dragonlance Bestiary     SAGA (no source link)

Hacklopedia Of Beasts     Hackmaster original/official source

Monsters Compendium     Heroes Of Adventure (no source link)

Godforsaken     Cypher System (no source link)

QUERP Bestiary     QUERP original/official source

Beyond The Black Sea Bestiary     Beyond The Black Sea (Dark Stars) original/official source

The Monster Overhaul     OSR (no source link)

Lairs And Encounters     Adventurer, Conqueror, King (no source link)

Baby Bestiary 2     Any original/official source