The Tau

the Tau are an alien race, inhabiting a small but dense region of space on the eastern edge of the Milky Way Galaxy, roughly 300 light years in diameter. The Tau were first introduced to Warhammer 40,000 in late 2001, the result of Games Workshop's plan to introduce a new race to the game.
The Tau have evolved and advanced rapidly since their first encounter with the Imperium of Man in the 35th millennium, rising from a hunter-gatherer level of technology to a starfaring race in less than six thousand years. Tau society has also advanced rapidly, from warring tribes to a unified caste system working towards common goals, known by the Tau as Tau'va, The Greater Good.
As well as the five castes of the Tau, multiple alien species are incorporated into the Tau Empire; the most significant of these being the Kroot and the Vespid.

The Kroot, while humanoid in appearance, are believed to have evolved from a carrion-eating avian form, as they possess vestigial beaks and a light bone structure. They have four digits on their hands and feet. While tall and wiry, they have an extremely strong musculature, coupled with quick reflexes, and are stronger than humans of equal size and weight. Their heads are crowned with tough quills that form part of their sensory apparatus; these connect to the brain via ganglia. Kroot eyes have no pupil, are generally milky white in colouration, and can see into the infrared spectrum, allowing Kroot to track their prey by body heat.

The Vespid are a highly evolved insectoid species from the planet Vespid, a gas giant three light years to the galactic south of the D'yanoi sept. The violet hued, storm wracked stratosphere of the world provides a shallow layer within which life, against all odds has evolved. They live on floating mountains held aloft by the lighter-than-air gases trapped within air pockets. They possess reasonably advanced technology but no interstellar travel.
They use a 'neutron blaster' which is designed to be a Space Marine killer. The blaster uses Tau technology combined with a type of crystal that only the vespid females can venture down to the planets surface to collect, that are unique to the planet Vespid.