The Rashan are speculated to have been created by the Old Ones during the War in Heaven. At this point, the C'tan had allied themselves with the Necrons against the Old Ones, and had beseiged their last bastions. In desperation, the Old Ones manipulated life into new forms with an ever stronger link to the Warp as potential warrior races with the ability to channel psychic power to defend themselves. This is speculated to have included the Rashan, along with the Eldar, the K'nib, and many others.
The Rashan were annihilated by the aftermath of the War in Heaven, after which the Eldar penned the Chorale Lilcartha, one of the oldest pieces of ancient Eldar literature known millenia later in M42.