Armed Phenomenon

The Armed Phenomenon (アームドフェノメノン) is a transformation in the Mega Man Zero series that allows the Four Guardians to increase their strength.

Characteristics
The transformation is very quick, taking only a few seconds: energy builds up on activation, causing the user to glow with a round aura that expands into a flash, obscuring a near instantaneous change. The Four Guardians trade their humanoid bodies for huge, animalistic vehicles armed to the teeth. The armed forms of the Four Guardians reflect their nature, with Fefnir becoming a reptilian tank, Leviathan becoming a submarine resembling a manta ray and Harpuia becoming a raptor/jet hybrid. They look nearly as mechanical as the Mutos Reploids, but their original faces are retained. The users of Armed Phenomenon can revert to their original forms voluntarily. There are no known risks or detrimental effects in the use of Armed Phenomenon.

Mega Man Zero 2
After a rogue Elpizo intrudes into the very center of Neo Arcadia: Yggdrassil, and damages the Dark Elf's seal created by Cyber-elf X, Zero sets out to reach Yggdrassil through the temples of the surviving guardians. Fefnir and Leviathan activated their Armed Phenomena to fight Zero in the Temple of Flame and Temple of Ice respectively, but even their powered up forms are no match for Zero. Although Harpuia attempts to take out Elpizo instead, he is forced by the Baby Elves Crea and Prea to transform and fight Zero as well in the Temple of Wind. Harpuia is also defeated, but he returns to his senses and pleads Zero to protect X in his behalf.

List of phenomena

 * Raptor Harpuia: After being possessed by the Baby Elves, unwillingly absorbs their power and takes on the form of a massive bird-like aircraft. Like regular Harpuia, his Armed Phenomenon form has terraforming capabilities, which gives it the ability to emit electricity and create strong winds. It also has on-board missiles, and often tries head-on attacks as well.
 * Monster Leviathan: Fairy Leviathan takes on the form of a manta ray submarine. She is much more aggressive, despite having lost much evasiveness due to the increase in size that this form gives her. In this form, her attack patterns change completely, she can now launch two ice dragons, and they can either chase Zero, or lock him into an ice box that she will later break. Also, in this form, she can launch ice beams from her forehead that will become ice blocks, and later float away, and shoot ice from her forehead, that will ricochet against the ground. She can also launch mini torpedoes that follow her through the room and later have a pattern of their own.
 * Dragon Fefnir: Takes on the form of a dragon tank. He uses its large size to his advantage, slowly and steadily forcing Zero to keep moving through a hall during his battle in his Armed Phenomenon form. He uses his four cannons to keep Zero on his toes.
 * Demon Phantom: Becomes a large bat-like creature. His is the only Armed Phenomenon that is not fought in the second Zero game, due to his death in the first game. Other concept art shows that some of Phantom's attacks in this form were planned to include dashing at high speeds and firing the long spear-like appendages at Zero (see here).
 * Seraph X: Copy X, the former leader of Neo Arcadia, takes a seraph-like appearance in his second form in Mega Man Zero. Most of his attacks in this game are also given an angelic theme to them, such as his imprisoning halos. Early concept art shows that Copy X's second form was originally planned to be much larger, featuring a huge drill on one arm and much smaller "wings". However, he does not revert to his original form on defeat like the Four Guardians do. He tries to transform again in Mega Man Zero 3, but is interrupted went the attempt triggers a trap placed in his body by Dr. Weil.

Similar Phenomena
Besides the Armed Phenomenon, there are also unnamed transformations similar to them, but whose uses are functionally the same. They are not classified as true Armed Phenomena because they are not pre-programmed for future use, and thus they must be obtained. The transformation involves the power of the Dark Elf or the Baby Elves, and the user is invigorated with dark power, but may find difficulty in controlling themselves as it requires the fusion of two consciousnesses.
 * Dark Elpizo: When Elpizo finally released the Dark Elf by destroying the Original X's body, he fused with her and became a more powerful version of himself that looks like a knight. When he was defeated by Zero, he then unleashed the true power of the Dark Elf upon himself, merging with her and taking the form of a dark winged monster. This form bears a striking resemblance in appearance to that of the Dark Elf herself, and in this form, the Baby Elves assist their mother and Elpizo in combat.
 * Demon Pantheon Aqua: During Childre Inarabitta's stage in Mega Man Zero 3, a Pantheon Aqua is modified by the Dark Elf in an attempt to halt Zero's progress through the sunken city area as she continues her escape. Despite its increased power, it is still only a Pantheon, and is easily dispatched by Zero.
 * Fusion Omega: Omega's third form is a giant Reploid which appears to be a combination of X, Zero, and the Dark Elf. This form is so large that it is impossible to see the whole thing on the Game Boy Advance screen at once. It features a huge cannon (modeled after the X-Buster) and a saber (modeled after the Z-Saber) which is never used in Omega's attacks; this is most likely because the saber itself was too long to fit on-screen. The middle head, Omega's original head, can also fire a red energy ball that can trap Zero.

Trivia

 * An Armed Phenomenon was designed for Phantom; however, his demise in Mega Man Zero prevented its use in the sequels.
 * The Four Guardians never consider using their Armed Phenomena against Omega.
 * Instead of using Armed Phenomenon in the manga, the Four Guardians merge into Cial's "perfect Reploid". Although this does not necessarily mean they cannot transform individually, this remains unconfirmed since the perfect Reploid was destroyed shortly by Zero.