Dracopedia Bestiary: An Artist's Guide     Any original/official source
The Monsters Know What They're Doing     Dungeons And Dragons original/official source
Angry Golem (web)     D&D 5e original/official source
Pathfinder 1e Bestiary 1     Pathfinder original/official source
Mythic Monster Manual v1     Pathfinder original/official source
All Sorts Of Critters (web)     Pathfinder original/official source
Monster Manual     D&D 5e original/official source
Monster Ecology Anthology     D&D 5e (no source link)
Dragon Magazine     D&D 2e/3e (no source link)
MojoBob (web)     D&D 2e original/official source
D&D 2e Monstrous Manual     D&D 2e original/official source
Scruffy Grognard's Monstrous Manual     D&D 2e (no source link)
13th Age Bestiary     13th Age original/official source
Fantasy Bestiary     GURPS (no source link)
Baby Bestiary 2     Any original/official source