This is an enormous Core-X that Samus discovered downloading the Varia Suit data from the download room. After she witnesses this, it retreats to an adjacent cavern, where Samus is then forced to destroy it to recover the data. Like regular Core-X, it is only able to attack via ramming, but it has several orbiting X parasites that will absorb several shots. Unlike standard Core-X, it is vulnerable to the Charge Beam. In fact, missiles are harmless to it. As damage is dealt to it, it becomes more reddish in color and begins moving around the arena more faster. After enough damage is dealt to it, it sheds its out casing and becomes a standard Core-X.
The shallow water in the bottom of the arena can pose a major hazard, as Samus lacks the Gravity Suit at the time, so at it sweeps across, she cannot effectively dodge its attacks.
After the battle ends, she naturally regains the Varia Suit, and as such allows her to enter superheated rooms. A difference between this model of the Varia Suit and it as it appeared in other games, as it also offered her protection from cold as well, since she became vulnerable to cold environments as a side effect of her Metroid vaccination.