Below is a list of all transformations/alternate states in the Metal Slug series.
When you consume around 16 food items (bananas, fish, cabbages, watermelons, carrots, etc.) your character will become obese. The announcer will note the transformation with the words "Woah! Big!" Becoming fat will increase your firepower but slow your movement. All weapons will change appearance to a comical pistol with a wide, bell-shaped barrel (with the exception of Two Machine Guns, where you will instead get a pair of comical submachine guns carved out of wood). The bullet size of all weapons is increased, but certain weapons have slower firing speed. All other weapons are also changed to a more comical appearance. Instead of a knife, you now use a fork and whip in close combat. The Fat transformation will wear off over time (not including time spent in vehicles), although one can return to normal immediately by picking up a diet pill or keg of diet powder.
IF you are directly hit by a zombie/Big Jun or come in contact with their viscera, your character will become a zombie. This transformation includes having your skin turn blue, being restricted to your default pistol weapon, and reduced movement speed via a shambling gait. Jump height is severely reduced to a small hop, although hopping repeatedly will help you move faster. You are also unable to crouch or obtain special weapons (except for the Thunder Cloud). However, your grenade attack is changed to a deadly, bloody, vomit attack which unleashes a giant wave of blood from your mouth which covers nearly all of the screen in front of you and can kill most enemies instantly. You also become invulnerable against most rebel weaponry, although another zombie attack and attacks from level bosses will kill you instantly. Zombie transformation can only be ended by picking up a cure pack, a square backpack conveniently marked with a red cross or after you stay as a zombie for a long time, it will wear off itself
If you are hit by purple smoke breathed by any mummy variant, a mummy's rolling bomb, or a purple cauldron dropped by a bat, you will be turned into a mummy (it should be noted that the locusts from the locust belching mummy will kill you rather than mummify). All characters retain their headgear and facial accessories for easy identification. As a mummy, you move slower, can only use the default pistol weapon and regular grenades, and suffer a delay before throwing grenades. Because of the focus on precision jumps in most mummy-filled areas, the mummy form compensates for slow speed and a short jump delay with increased jump height and longer air time. Unlike the zombie form, the mummy form can still crouch, but unlike the zombie form the mummy form does not gain immunity to any type of attack. The mummy form can only be cured with a mummy cure, a small crystal glass vase filled with multicolored liquid. The mummy form is the most disadvantageous transformation (barring Snowman), as it carries multiple downsides with virtually no benefits. If you attempt to throw a bomb while crouching as mummy form, the character will throw his/her head instead of the Grenade, both of which have the same effect.
When you are hit by a yeti's ice breath or a blue flying pod, you will turn into a frozen snowman. As with the mummy form, each character's snowman retains their head-gear for easy identification. Snowman form is less of a transformation and more of a state of immobilization, as you are completely defenseless when trapped in a snowman while enemy yetis close in to kill you with their bone clubs. Luckily, rapidly pressing buttons and wiggling the joystick will free yourself from the snowman state posthaste.
If you get hit by a bullet from a scientist's rifle you will be turned into a small monkey wearing a diaper (with character retaining their headgear and shirt color, much like the mummy and snowman). The monkey form has exceptional jumping ability, but reduced ground speed, and restricts the player a small Uzi for a weapon, which is automatic but with a slower fire rate compared to the Heavy Machine Gun. In monkey form, you can grab and climb across rails along the ceiling to fire down on enemies. The monkey form can be cured with a green serum bottle. The monkey form is based off the sprite of Utan.