Horus Rebellion

The Horus Rebellion was an intense skirmish mentioned by GFS Olympus crewmen in Metroid Prime 3: Corruption that is compared to the Space Pirate upcoming invasion of the Pirate Homeworld. All that is known about it that it was a rebellion by an unknown people group. It is spoken of as though it was a significantly difficult battle for the Galactic Federation that cost them many fighters.

Horus is assumed to be either the planet the war fought on or the name of the alliance or ethnicity of the rebels. It is unknown when it took place or why, aside from the fact that it was a rebellion.

The only mention of it in all of the Metroid series is at the beginning of Metroid Prime 3: Corruption, when Samus is listening in to a conversation by several Male Fleet Troopers. It is impossible to gain any more information from them, as speaking to them will simply cause them to dismiss her, citing their duties as the cause.

Trivia
In Egyptian mythology, Horus was the son of Osiris and Isis. He was god of the sky, with the sun as his right eye and the moon as his left, and was represented as a falcon-headed man. He was a warrior and a hunter, and held a long time rivalry with Set, the god of the desert and the murderer of Osiris, which culminated with many battles to gain the rightful throne of Egypt.