A Core-X is a type of X Parasite found in creatures more powerful than those inhabited by most X. This X will remain intact even after Samus defeats its host due to a hard shell that can only be cracked with Missiles. Once Samus breaks through the shell, she may absorb the X regaining one of her latent abilities in the process.
They will attack by ramming.
A Hard Core-X is an X in possession of a beam weapon that they may fire from an eye. Unlike regular Core-X, a Hard Core-X is only vulnerable when it is preparing to fire its beam weapon and may only be damaged by a direct hit to its eye. Furthermore, when its eye is hit with a Missile, the Hard Core-X will fire its beam attack.
- "The other method of reclaiming lost abilities is to absorb a mega-sized X parasite known as a Core-X. The image here is one of several types of these dangerous creatures. The abilities of the Core-X parasites differ from type to type, but the thick thorny membrane that encases them is a common feature. This tough casing must be ruptured by a powerful attack, such as a Missile blast, to render the core vulnerable to absorption. Some of the Core-X parasites are incredibly powerful- engage them with caution. A Core-X will release X parasites when shot with beam weapons. Try using this to your advantage when low on Missiles or energy." Metroid Fusion Instruction Booklet, Pg 29