The Kroot are a race of avian-humanoids from the Eastern rim, deep within the space controlled by the Tau Empire. On their homeworld of Pech, they grew to be strong hunters with a tribal society, lead by their Shamans. Once the Tau Empire gained the ability to sail the stars and discovered that there were many races much larger and more powerful than them, they decided to look for help. They came across Pech and offered them a trade, to help advance their tech in exchange for numbers. They made a pact and the Tau gave them the information to create their own stellar crafts. The Kroot could now sail the stars and the Tau would have a force of brutes and hunters that would give a Space Marine a run for his money on even terms.
Many years have passed and the Kroot have grown in size. With the ability to travel far from Pech, they were able to settle worlds and expand their race, as well as add many more Kindreds to their species. Slowly, they eventually encountered the Imperium, where they were met with mixed results. Many times, they were fired upon on sight, as most Kroot fleets are under the use of the Tau Empire. Occasionally, they ask to trade. These are often denied, but a Rogue Trader with a keen eye for a good deal would make these trades, sometimes even hire them for life-contracts as bodyguards and such.
After many centuries of these types of dealings, Kroot were found in many places in the Imperium. Eventually, they even arrived in the Segmentum Pacificus. And like many other similar tribes, many wanted to settle and make a planet of their own. In the Kronous Frontier, they were allowed such rights, as long as they paid dues and openly traded with the rest of the Imperium, of course. Amongst all the Xenos races, the Kroot were the first to be openly acknowledged as an ally to Humanity, though mainly through contracts and material gain. In the Koronus Frontier, they willingly choose to colonize some of the most inhospitable Death Worlds, as they prove to have the strongest fauna, best for improving the species. And much of the Imperium is happy that they will clear the way for any resource harvesting they can aide in.