Arachnus is a tough shelled creature who is encountered in both Metroid II: Return of Samus and Metroid Fusion. In Metroid Fusion, it was infected with the X Parasite, and became known as Arachnus-X. In Metroid II, it was an optional boss fight, and it was the guardian of the spring ball. When encountered on SR388, he was vulnerable only to morph ball bombs.
In Metroid Fusion, after it was infected by the X, it developed several new abilities, such as the ability to project a sonic wave. Here, he will repeatedly try to roll over Samus, but will also spit fireballs and fire the aforementioned sonic waves at her. Its underbelly is vulnerable to missiles and standard shots from the power beam, making it much less of a threat than it was on SR388. After it is killed and the Core-X has been cracked open, Samus is able to recover her morph ball ability.