Tarellians

Tarellians are a minor reptilian species of xenos found throughout the galaxy, but are of little consequence to its affairs. They are commonly known as Tarellian Dog-soldiers, because of their snouted faces and low opinion most other species have of this broken mercenary race. Physically, Tarellians are narrow-waisted, broad shouldered aliens that are slightly shorter than most humans.

History
They possess a zealous hatred for humanity above all else. A hatred humanity is well deserving of, as the Imperium led a series of exterminatus attacks on the Tarellian homeworlds during the Great Crusade. Nearly wiping out their species. The Tarellians never fully recovered from the xenocide. Robbing them of their potential as a species. The very latest blow to them is the destruction of a Tarellian civilisation that was devoured by Hive Fleet Moloch during its advance from the galatic north.

In more recent times, Tarellians have been frequently sighted fighting for the Tau Empire. Not conquered or annexed by the Tau, they are simply a new and prospective client for the Tarellians. As such the relationship between the two races is more similar to what the Tau have with the Kroot.