Arachnus

Arachnus, plural form Arachni, are a species of creatures found on SR-388. At the near-top of the food chain, it has proven to be a very fearsome creature.

In Metroid Fusion, Samus encounters an Arachnus in the Main Deck. It attacks her either by spitting flames along the floor and rolling down them or by using his Sonic Wave attack. To dodge the flames, she must hold onto the ledges on either side. If he hits the wall whilst rolling and she is hanging on, Samus will be knocked off and into his grasp. She cannot avoid the Sonic Wave attack by hanging onto a wall, though. To defeat it, she must use Missiles to hurt the front part of his body, where the shell isn't. If she were to run out of Missiles, then normal beams will work, but it will take a longer time. Once he turns into a X|Core-X, he will send out a few yellow and green X|X Parasites that will give Samus more missiles if she doesn't have any and replenish her stamina. Once Samus defeats the Arachnus, it will give her the Morph Ball ability.