Reek

The Reek, also known as the Noisome Reek, are a xenos species living in the Eastern Fringe, and are the punching bag of the whole area, and that's no small feat. Despite the fact they're trying to create their own little galactic empire in the area, they show remarkable levels of fail while doing so: not only they attacked planets of both the Imperium and the Tau Empire at the same time (both dwarfing the Reek in technology, firepower and manpower to a ridiculous extreme), but when they confront their military forces, they get absolutely stomped. During the Battle of Ghallenburg, a single Rogue Trader was able to keep them from taking the planet by attacking them while the Reek voidswarms were entering the planet. They are also one of the few alien species the Tau actively hate. In the 4th ed. Tau Empire Codex, they are described as a hateful species that attacked the planet Sir'coa, and were once again curbstomped by the Fire Warriors led by Aun'Va.

Their spacecrafts are called voidswarms, so we can assume they are some sort of insectoid aliens, but this is unclear, as descriptions of them are quite rare, and I don't think there is any form of art of them. Also, they have a hideous stench, according to their name.

Notable Species of Warhammer 40,000
Major: Eldar, including Dark Eldar Humans, including Abhumans Necrons Orks Tau Tyranids
Minor: Ambull Araklionid Barghesi Banelings Bale Childers Drahendra Caradochians Cimmeriac
Cryptos Cythor Fiends Demiurg Donarathi Drugh Dracoliths Drax Enoulian
Enslavers Fra'al Galg Gykon Hrud Jokaero Jorgall K'nib
Kathaps Khrave Kinebrach Kroot L'Huraxi Lacrymole Laer Lelith
Loxatl Medusae Megarachnids Nekulli Nicassar Old Ones Q'Orl Rak'gol
Rangda Reek Reptos Saharduin Saruthi Scythian Simulacra Slann
Slaugth Sslyth Stryxis Tarellian Thexian Thraxian Thyrrus Tushepta
Umbra Ur-Ghul Vespid Watchers in the Dark Whishperers Xenarch Yu'Vath Zoats