Call to Arms
Where the magic of the Fey leaks into the material world, intense negative emotion sometimes manifests as a monster. This is a world where a child’s fear can literally spawn a monster under the bed.
Many of the humanoids listed in the Monster Manual enter the world this way. Orcs, goblins, ogres and the like are not born, they are spawned by fear, hatred and greed. Many other truly monstrous creatures fall under this rule, too.
These monsters incarnate with real, physical bodies often in a dark place near the source of emotion that created them. They may sulk off into the world, hiding while they gather in greater numbers, possibly stealing or even forging weapons and armor, or they may immediately dash about causing mayhem.
Devils, demons and celestials have similar origins. Though some are created through specific rituals, it is still using wild Fey magic to incarnate human passions. The main difference is these powerful incarnations can be banished to the Fey, although they are not truly Fey, are not immortal and can be destroyed on any plane like other mortal creatures.
True giants are mortal races, originally humans enhanced by the God-Emperor to serve as his enforcers. Occasionally a nightmare giant will spawn which is not of those races (after all human fears often take a humanoid form) but such monsters have no place in giant society. Ogres, trolls and other twisted monsters marked as ‘giant’ in the Monster Manual are spawn, not mortal creatures related to true giants.
Dragons and Kobalds are mortal creatures created by the World-Wyrm. Metallic dragons came first, but some were corrupted by Kyrios, giving rise to the chromatic breeds.
Elementals are semi-sentient beings spawned spontaneously by the Magic of Creation still at work in the Material World, which (unlike Fey magic) reacts with inanimate matter and raw elements. Dwarves and Gnomes also come into the world this way, though they are fully sentient and tied to a clanhome.