The Slann were a reptillian or
amphibian species who were servants of the Old Ones though current fluff
states that they in fact are the Old Ones. The Slann are reputed to have
created some of the major races, the most notable of which are the Orks (who
refer to them as the brainboyz) and Eldar (who are the only race that keeps
records from the age) who were created with their psychic abilities to take
part in the war between the Necrontyr and the Slann sixty million years ago
in which they were all but wiped out by the C'tan and the Enslavers plague.
The Mon-Keigh (Human) race was a race that basic evolution was allowed to
mould without the guidance of the Old Ones.