Tome Of Adventure Design

The Creature Crafter (system agnostic)

Dragonlance Bestiary (SAGA system, monster abilities in the prologue)

GiffyGlyph for D&D 5e

Heroes Against Darkness

Ironsworn Delve

Mythras ("Creatures" chapter)

Worlds Without Number

Knock #2

Five Torches Deep

Nihb's simple yet versatile monster generator