Mon-keigh

The Mon-keigh were an ancient xenos race that was exterminated by the Eldar millennia before their first contact with Mankind.

History
The Mon-keigh are described in Eldar stories and legends as a race of sub-intelligent, cannibalistic beasts that lived in the twilight realm of Koldo. These misshapen monstrosities invaded the Eldar lands and subjugated them for many years, until they were eventually cleansed from the galaxy by the hero Elronhir.

Since then, Imperial scholars surmise that the word "mon-keigh" of the Eldar Lexicon refers to any non-Eldar species they deem inferior and in need of extermination - generally, Mankind.

Source
Mon-keigh
 * Codex: Eldar (3rd Edition), pg. 48.