Collosun is a recurring boss in Metroid Prime: Federation Force. It is first faced in M05: Excavation on Bion, attacking the Federation Force. Another Collosun is encountered in M09: Blender, if the Force decides to collect a certain MOD, and the final one during M20: Insurrection.
Collosuns appear to be mechanoids created by the ancient Bion race to guard valuable items (much like the Chozo-made Torizo and Alimbic Guardians), though this is unconfirmed and they may be something else entirely.
This first Collosun is faced when the Federation Force is sent to retrieve a data codex. At first, it appears to be an ordinary statue, but upon the Federation Force entering its chamber, the boss seemingly "awakens"; its torso starts shining, it gains a large head, and it jumps up and roars at the intruders.
Collosun attacks the Marines by spinning and teleporting as well as launching green projectiles at their Mechs. It is vulnerable to any weapon, but is best fought using the nearby Cannon. After the Force damages it sufficiently, the boss suddenly disappears, allowing them to continue exploring the Bion ruins.
In this later mission, the Federation Force is sent to retrieve the power cells. The encounter with a Collosun during this mission is optional, instead only provoked if they collect a MOD in a small cave. This battle differs very little from the previous one otherwise, although there is no Cannon to aid the Force in battling the Collosun.
The final Collosun is fought after the Force, on their mission to destroy the Bion Transfer Device, recover their Mechs. It fights along with several Elite Pirates, guarding the room containing the Transfer Device.
The name Collosun appears to be a corruption of the word "Colossus" in Ancient Greek, which refers to a giant statue, fitting the Collosuns' statue-like appearances. The name Colossus was also used for a Space Pirate ship in Metroid Prime 3: Corruption.
Alternatively, Collosun may be derived from "Colossal", which would simply be referring to the boss' large size.