The Brain Boyz are believed to be an ancient subspecies of Greenskins that are related to the Orks and were believed to be a master or ruling caste of their kind who shared a similar physique with their cousin species.
Ork legend (such as it is) is generally passed down by the Runtherds. They speak of a legendary caste of greenskins who created the Orks of today. This ancient race was extremely intelligent, and held dominion over the Ork castes. They were much smaller than their servants, so they bred the Orks to be as strong and fierce as possible to protect them from predators and invaders.
It was believed that this group was the driving force behind their civilisation and were responsible for developing their technology as well as directing the different castes. There is even some speculation that both Orks and Gretchins may have been specialised mutants that were deliberately created by the Brain Boyz to serve certain duties, though it believed that the truth of this will never be known for certain. Regardless, it is known that the Brain Boyz died out long ago and the Orks inherited their civilisation to the point that this ruling caste become little more than legend. According to Orkish beliefs, the Brain Boyz died from a great plague which ran across many centuries that caused their numbers to dwindle and eventually led to their extinction. It is believed that this caste was aware of their falling numbers and took steps to preserve their knowledge by engineering it into the genetic structure of their slaves. This lasting legacy allowed the Orks to retain their high level of technology in later years by way of artificial strands of DNA which contains knowledge imprinted in their metabolism that allowed the Orks to live following the demise of their masters.