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.
This article or section includes speculation, observations or opinions possibly supported by lore or by Blizzard officials. It should not be taken as representing official lore.
Though the Monster Guide calls these orcs 'fel orcs', there seems to be some level of distinction between them. 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. It can be assumed that with continued exposure to the corruption from Mannoroth's blood they may have mutated into the more physically demonic fel orcs (and the process would likely be irreversible). 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. 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. There is also a chance that the difference is related to the possibility that "fel orcs" as we know them were not conceived by Blizzard until Warcraft III: The Frozen Throne.