The Nemesis Alpha is first seen in the Resident Evil 3 remake as an enemy. They act similarly to the Plagas of Resident Evil 4 where they latch themselves onto their hosts' head and take over their bodies.
They serve as both a weapon and an "armor" of some sort in the Zombies' head. As an offensive capabilities, they can use their elongated tentacles to lash out their victim or grab the player character to swallow their heads with its gaping maw. Alternatively as a defense, they protect the zombies' head with their impervious limbs, having high resistance against firearms.
The best way to destroy the Nemesis Alpha is to first target their host zombie's body. Enough damage will cause the Nemesis Alpha to expose their weak points.