Dracopedia Bestiary: An Artist's Guide     Any original/official source
Pathfinder 2e Bestiary 1     Pathfinder original/official source
Monster Manual     D&D 5e original/official source
Flee, Mortals     D&D 5e (no source link)
Monster Ecology Anthology     D&D 5e (no source link)
Scruffy Grognard's Monstrous Manual     D&D 2e (no source link)