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 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 3     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)
MojoBob (web)     D&D 2e original/official source
Scruffy Grognard's Monstrous Manual     D&D 2e (no source link)
AD&D Monster Manual     AD&D 1e (no source link)
Dragonlance Bestiary     SAGA (no source link)
Hacklopedia Of Beasts     Hackmaster original/official source
Monster Guide     World Of Warcraft RPG original/official source
Manual Of Monsters     World Of Warcraft RPG original/official source
Mystara Adversary Compendium I     Dragon Age original/official source
Low Fantasy Gaming Companion     Low Fantasy Gaming original/official source
Bestiary Of Cryptofauna     Old School Essentials (no source link)
Iron Falcon Handbook Of Monsters     OSR (no source link)
Baby Bestiary 1     Any original/official source