The following is information for the Thanos: Titan Consumed novel.
When Thanos was exiled from Titan, he tried making it to the Kree homeworld of Hala. However, his ship ran out of fuel. While he struggled to survive he was found and rescued, although put into slavery. Using his genius intellect, Thanos eventually became a part of the captain’s inner circle as the ship’s engineer.
He eventually took over the ship and renamed it Sanctuary. Eventually he landed on Xandar and from Xandar got another ship to an Asgardian post on Alfheim. From here he entered a wormhole expecting to go to Asgard, instead he ended up on the Chitauri homeworld. Learning of Chitauri society, he learned they are a hive mind with characteristics of reptiles, insects, and cyborgs.
One of them, called The Other, was the only member not a part of the hive mind. Recognizing Thanos’ superior intellect, he asked Thanos to command the Chitauri as they craved conquest yet didn’t have the strategic or tactical mind to execute war. Using The Other as a conduit to communicate with the hive mind, Thanos became the ruler of the Chitauri.
Thanos used the Chitauri as both scientific experiments and practice dummies. Using his knowledge of science, he genetically modified the Chitauri warriors to become stronger. This also allowed his body to improve along with his fighting abilities. During this time as he prepared to return to Titan he learned he was too late, Titan had died as he predicted. Thanos would then go world to world trying to “save” it but end up killing half or all of the population.
After several years of conquest, he finally allowed the Chitauri a break from conquest, so they could settle on a new world that better fit their needs. Later when Thanos was bored, he decided to return to genetic engineering. Using samples of DNA from the Chitauri, his own DNA, and various samples of worlds he’d killed, Thanos created a new creature he called The Outriders.
He planned on using this to replace the Chitauri as he felt they were limited by their hive mind. The Outriders could also be programmed to be simple grunts for combat and nothing else. Thanos made millions of Outriders to help his military efforts, as seen during the Battle of Wakanda from Avengers: Infinity War.