Boss Arachnus

Boss Battle 3, also known internally as Vs. Arachnus, is a battle theme from Metroid: Samus Returns. It was composed by Daisuke Matsuoka.

Description
Boss Battle 3 plays during the boss fight against Arachnus in Area 2. It opens with a stinger, accenting the surprise of Arachnus revealing itself. In contrast with most boss themes in Samus Returns, this theme is more playful than threatening, although it still creates considerable tension with its dissonant electronic instruments. It can be heard here:

Boss Battle 3 may be based upon the original Metroid battle theme from Metroid II: Return of Samus, composed by Ryoji Yoshitomi. Most notably, the stinger at the start of the battle is nearly identical in both songs. The harmonic patterns in Boss Battle 3 are also reminiscent of the melody of the Metroid battle theme. This may have been a deliberate reference by Daisuke Matsuoka, although this is not confirmed. The Metroid theme can be heard here:

Trivia

 * Surprisingly, Boss Battle 3 is not a remix of VS. Mulcalah, Arachnus-X's boss theme from Metroid Fusion. Instead, VS. Mulcalah is remixed as Boss Battle 6 and played during the fight against the Diggernaut.