Koloss are large and dangerous beasts that the Lord Ruler uses in his army. They are Hemalurgically created, through the use of four hemalurgic spikes stuck through a human body at certain points. While Koloss are regarded as remarkably stupid and reckless, they are not. Koloss are simply unsophisticated, uncultured, and very straightforward. Koloss are also known for being uncontrollable, except by the Lord Ruler. This turns out to be a myth, as they can be controlled by incredibly powerful soothings. Vin is able to control a small army of them using Duralumin during the Siege of Luthadel.
Jastes Lekal is able to command a large army of them by paying them money, and using them to attack Luthadel. The reason for this is that the Hemalurgic spikes used to create them leak power when not in use. After the Lord Ruler's death, the koloss are not given a fixed amount of spikes each year, so they continue to reuse old spikes more than they normally would. This causes the koloss to act more human.
The older a Koloss becomes the larger it is, however its skin doesnt grow anymore, causing it to tear and rip in places, such as around the eyes, however a young Koloss's skin is baggy and droopy.
During the remaking of Scadrial, Harmony most likely modified the Koloss so that they can function as an independent species. This can be seen in Alloy of Law with the character Targus, who is said to be koloss-blooded. Koloss-blooded are also able to hold allomantic abilities, as he is also a pewterarm.