Human Core-X

From Wikitroid, the Metroid wiki

The X-Infected Scientist is found at the Sector 3 boiler's cooling fan Control platform. It looks like a relatively small human, but if you shoot it a few times, it will turn into a Hard Core X and start shooting Wide Beams at you. You have to kill it before the boiler explodes. Of course, it will give you the Wide Beam.