The Mon-keigh are described in Aeldari stories and legends as a species of sub-intelligent, cannibalistic beasts that lived in the twilight realm of Koldo.
These misshapen monstrosities invaded the Aeldari lands and subjugated them for many Terran years, until they were eventually cleansed from the galaxy by the hero Elronhir.
In general, it is now a pejorative term used to describe humanity.
- Codex: Eldar (3rd Edition), pg. 48