How about this idea. Orc's and all their lesser kin are a self replicating self reinforcing and adaptive genetically programmed Bio weapon. Designed to destabilize a world before invasion or conquest.
1) Designed by humans during the dark age of technology and gone wild.
2) Or far older and created by the Old Ones perhaps, and now gone wild.
Life cycle: Spores seeded by covert means,
Fungus grows from spores,
When enough fungus or other organics available, Snottlings begin to emerge.
When enough Snottlings to serve as food, Squigs emerge.
As Squig and nuisance Snottling population hit critical mass, Grots emerge.
When a large enough Grot population exists to serve as slave labour force
Once this point is reached your world is toast!