Slann
From Lexicanum
The Slann are a reptilian or amphibian species who once were servants of the Old Ones. On orders of the Old Ones they are reputed to have fostered 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 some records from the ancient past) whose psychic abilities were fostered to take part in the war between the Old Ones and the C'tan sixty million years ago. They also created the Jokaero. The surviving Old Ones fell victim to the Enslavers. Humanity was a race whose basic evolution occured without any guidance of the Old Ones.
Older version
"Rick Priestley background" states that the Slann are the Old Ones. These original Slann are known as the Old Slann, and were an ancient and advanced race, the greatest of all spacefaring races. They guided the development of the Eldar to become the inheritors of their achievements. It's suggested Eldar and Mankind share a common ancestor. There is little physical difference between Eldar and humans, and it's said the two races are genetically close enough that it's possible for them to interbreed.
The Slann's history differs from most archetypal super-advanced civilizations which reach a golden age and then suffer a cataclysm which all but destroys their race. It's said no records exist of any destructive cataclysm befalling the Slann. At some point, the Slann retired from their role in galactic affairs, perhaps coming upon some great cosmic truth.
The Old Slann had developed the original Webway, which was later perfected by the Eldar when they inherited the Slann's role in the galaxy. As well as teaching the Eldar of the many positive aspects of the warp, the Old Slann are said to have forewarned the Eldar about the dangers that they would face. When the Fall came long after the time of the Old Slann, those few that had heeded the warnings survived.
The Jokaero are also suggested as being the possible creations of the Slann.
The Old Slann descendants survive into the current Age of the Imperium as the Slann, resembling humanoid frogs with a Mayan/Aztec like culture. Although no longer concerned with galactic affairs and existing in a much more primal state, most Slann colonies maintain their advanced technology and knowledge.
Either the Slann were the servants of the Old Ones, or they, and everybody else, are unaware that they were the Old Ones themselves in the distant past.
