Hrud
The Hrud are a race as old, if not older, than humanity itself. They were a creation taken over from the old Rogue Trader days and into modern Warhammer 40,000.
Initially, they were the Space Skaven, and garnered little attention. Then along came Xenology, an out-of-print Black Library book which detailed the findings of a Magos Biologis on his pan-galactic, semi-heretical travels to understand what makes aliens tick; mostly by probing, disemboweling, dissecting, and doing generic alien-abduction-type things to random xenos scum. In his travels, he encountered the Hrud, a jellyfish-like race of decaying spine-creatures, and that is how GW sees them now. The Hrud managed to avoid the ax, and skirted the edges of unpopularity to such and extent that GW actually gave them a face-lift as 40k became less Warhammer Fantasy in space and more grimdark, unlike their long-lost brethren the Squa- *BLAM*.