Few Hollows have the potential to remove their masks naturally, and even if they manage to do so, the change may not be significant. Sōsuke Aizen is able to artificially create Arrancar with the Hōgyoku, which also unlocks much more of their potential than a natural transformation would. The process of turning a Hollow into an Arrancar is called Shinigamification.
Arrancar and ordinary Hollows differ in several ways:
Arrancar assume a humanoid body, as opposed to the animal-like or monstrous forms seen from Hollows. Upon becoming an Arrancar, the chances of a Hollow retaining a humanoid form increase with their intelligence; only Vasto Lorde-class Menos will definitely assume a humanoid form. This new form will retain remnants of their past selves, like fragments of their removed masks and a Hollow hole. While some Arrancar prefer to conceal these remnants (such as Nnoitra and Halibel), others (like Aaroniero) hide their faces completely to escape unwanted attention. Arrancar can gain a Zanpakutō. Unlike with Shinigami, this Zanpakutō is not sentient, but simply their "true" Hollow form sealed into a sword. Therefore, an Arrancar's release of their Zanpakutō is merely a temporary restoration to their former selves, granting them any powers their Hollow form possesses. The act of release is called Resurrección (Spanish for "resurrection"), which is activated with a command phrase followed by the name of the sword. The Resurrección of Espada-level Arrancar releases enough power to be comparable to a Shinigami's Bankai. Few Arrancar do not have a Zanpakutō, being too weak to seal their excess power into a blade, while others prefer to have it constantly released, as with Iceringer and Demōra. The Arrancar under Aizen's command have a standard theme for their clothing. Their uniforms consist of a white jacket, black sash, white hakama, and black and white shoes that resemble sandals; in essence, the uniforms are basically inverted Shinigami uniforms.