Sigma Head

Sigma Head (シグマヘッド) is the first boss of Mega Man X5. He wounds the character opposite to the one the player chooses (he'll destroy X's Force Armor if the player chose Zero, or destroys Zero's buster if the player choses X) and then hides inside a statue under construction. It's a giant head of Sigma that releases small energy spheres from its mouth or energy spheres from its eyes. He can also attack by ramming the character across the screen. After losing half of his energy, he will also fire large energy beams from his mouth. His only vulnerable area is the cannon's lens that only appears when he opens his mouth to attack. After this boss is destroyed, Sigma's real plot is revealed.

After X or Zero have defeated him, they discover that Sigma lost to them on purpose so that he could cover the entire Earth with the Sigma Virus with the resulting explosion. 

Other Media
The Sigma Head makes an appearance in the Rockman X (manga), in a role similar to the game only that X and Zero fight it together. It even does the same thing after its defeat from the game (Which is covering the entire Earth with the Sigma Virus).

Similar enemies

 * Sigma Virus