The Monsters Know What They're Doing     Dungeons And Dragons original/official source
The Monsters Know What They're Doing 2 (moar)     Dungeons And Dragons (no source link)
OSE Encounter Activity Tables     Any original/official source
Pathfinder 1e Bestiary 1     Pathfinder original/official source
Pathfinder 2e Bestiary 1     Pathfinder original/official source
Monster Codex     Pathfinder original/official source
Classic Monsters Revisited     Pathfinder original/official source
Monster Manual     D&D 5e original/official source
Flee, Mortals     D&D 5e (no source link)
Ultimate Bestiary: Revenge Of The Horde     D&D 5e original/official source
Volo's Guide To Monsters     D&D 5e original/official source
Monster-A-Day Compendium     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)
Dungeons And Dragons Essentials: Monster Vault     D&D 4e (no source link)
Dragon Magazine Monster Ecologies     D&D 2e/3e (no source link)
Scruffy Grognard's Monstrous Manual     D&D 2e (no source link)
AD&D Monster Manual     AD&D 1e (no source link)
Midgard Bestiary     13th Age (no source link)
13th Age Bestiary     13th Age original/official source
Dragonlance Bestiary     SAGA (no source link)
Hacklopedia Of Beasts     Hackmaster original/official source
Monster Guide     World Of Warcraft RPG original/official source
QUERP Bestiary     QUERP original/official source
Mystara Adversary Compendium I     Dragon Age original/official source
The Monster Overhaul     OSR (no source link)
Baby Bestiary 1     Any original/official source