Chaos orcs are orcs that drank from Mannoroth's Chaos Well in Ashenvale Forest and became corrupted.They were orcs of the Warsong clan that followed Grom Hellscream's decision to drink from the well to gain power in order to defeat Cenarius and his forces (Shadowleaves, Moonhunters, and Nightblades). By drinking from the well, they were further corrupted with more of Mannoroth's blood, turning red and growing larger.
Chaos orcs also fought under the banner of the Burning Blade during the First War. Some of the orcs who passed through the Dark Portal were driven mad, becoming chaos orcs and joining the Burning Blade.
Known chaos orcs
In the RPG
Chaos orcs, also known as fel orcs, are the creation of Mannoroth, who "blessed" the orc race by infusing within them fel energies and binding their will to the Burning Legion. While Grom's sacrifice freed most of his race, many orcs remain loyal to Mannoroth and worship the demons, gaining tremendous strength in exchange for their service. While not all demon-worshiping orcs are fel orcs, more than enough still partake of demon blood for the strength it grants them.
Speculations and theories
There seems to be some level of distinction between chaos orcs and fel orcs.
Grom's chaos orcs did not show the level of mutation of the fel orcs in Outland. They had not yet grown spines or bony structures from their backs and arms like those fel orcs. They also lacked the six long and skinny fangs of further mutated fel orcs. Other than being red, chaos orcs looked like regular orcs and could revert back to regular orcs, for example with Grom it was the help of a special ritual of orcish and elven magics.
It can be presumed that with continued exposure to the corruption from Mannoroth's blood they may have mutated into the more physically demonic fel orcs. However, both are created from Pit Lord blood, and some fel orc mobs in Hellfire Peninsula just use the normal orc model colored red instead of the true fel orc model, like Warlord Morkh, who commands confirmed fel orcs. If this is not just a mechanic (for wearing helms, shirts, and such), it shows that there may be 'intermediate' fel orc stages that chaos orcs may be a part of.
This section concerns content exclusive to Cataclysm.
Red skinned orcs appear as one of the random Horde Portal-Sentries that are guarding the Dark Portal for the Horde along with the Alliance. The general goods vendor, Quartermaster Dekrok, also has a red-skinned orc model. They may be chaos orcs, or they may be fel orcs. Also, they might be another color of Mag'har, or this may be a simple skin error.
The Blackrock Slave Master.
As seen in Warcraft III: Reign of Chaos.
Grommash Hellscream mutated into a chaos orc.
Warlord Morkh, a fel orc with minor mutations