Monstrosities     Swords & Wizardry original/official source
Pathfinder 1e Bestiary 2     Pathfinder original/official source
Mythic Monster Manual v2     Pathfinder original/official source
Forgotten Foes     Pathfinder original/official source
Critter Compendium     D&D 5e original/official source
Monster-A-Day     D&D 5e original/official source
MojoBob (web)     D&D 2e original/official source
AD&D Fiend Folio     AD&D 1e (no source link)
Adventures Dark And Deep     Adventures Dark And Deep (no source link)