Wario's Different Forms
Wario's various forms throughout the series. These range from powerups to simply conditions caused by enemies. From Wario Land 2 through 4, Wario can have several status effects inflicted on him that are sometimes helpful for reaching certain areas, but are otherwise general nuisances in Wario's quest. Wario: Master of Disguise returns Wario to his roots, now possessing a variety of powerups activated by the player.
Wario Land
Tiny Wario
In 'Wario Land' and 'Wario Land VB', Wario would become tiny when damaged by an enemy. As Tiny Wario, he could no longer use the body slam attack or pick up and throw enemies. Finding any pot would return him to normal size or Power Up, depending on which he grabbed.
Tiny Wario reappears in 'Wario Land 2' as a transformation, caused by the spell-casting Gooms in some levels. While tiny, Wario could jump higher, and would return to normal when damaged or if he fell into water. Tiny Wario also appears with his own character level in Wario Ware: Smooth Moves when a motorcycle build by Penny Crygor turns Wario into hundreds of Tiny Warios.
Bull Wario
Dragon Wario
Jet Wario
Virtual Boy Wario Land
Sea Dragon Wario
King Dragon Wario
Wario Land 2, 3, and 4
Ball o' String Wario
Getting covered in Wormy or spider silk turns Wario into Ball o' String Wario, a rolling yellow yarn ball in 'Wario Land 3'. Ball o' String Wario will roll back and forth across a course, bouncing off walls, until he eventually unravels back into Wario. As Ball o' String Wario, Wario can break through some blocks and walls. This seems to be a combination of Snowman Wario and Flaming Wario.
Bouncy Wario
Getting hit by a hammer in 'Wario Land 2-4' will turn Wario into Bouncy Wario, a springy version of himself used to reach new heights. In all three versions, Wario has a time limit before he will return to normal (hitting a spike will also work). However, in 'Wario Land 2 and 3' pressing A will let Wario jump slightly higher, while in 'Wario Land 4' pressing A will cause Wario to jump up until he hits an obstacle (usually the ceiling), and then return to normal. This is the only transformation used during a final battle, specifically against the Golden Diva in WL4.
Bubble Wario
If Wario runs into a bubble while swimming underwater in 'Wario Land 2-4', he will become trapped inside and float toward the water's surface, only able to move slightly to the left or right. However, Bubble Wario is also able to float against currents. When Bubble Wario reaches the water's surface, or runs into an obstacle along the way, he will be released. This is the only one of Wario's transformations that can be used in conjunction with water.
Crazy Wario
When Wario gets hit by a ball thrown by a penguin in 'Wario Land 2', he becomes Crazy Wario. While it is hard to control Wario, he can burp at enemies to defeat them. Falling into water or being damaged will cure him. Crazy Wario also appears in 'Wario Land 3' if Wario is hit by a certain enemy, though he can no longer burp. In this case, it is a complete inconvenience...
Electric Wario
If Wario is hit by a certain bug's spit, he will temporarily become Electric Wario in 'Wario Land 3'. Electric Wario instantly falls to the ground, bounces a bit, and stands there shocked before regaining his composure. Electric Wario isn't really useful for anything, except that enemies will be destroyed if they run into him while he's charged.
Fat Wario
Pigging out (more than usual, anyway) on stuff like cake, doughnuts, and apples will turn Wario into Fat Wario in 'Wario Land 2-4'. As Fat Wario, Wario throws his weight around to defeat any enemy just by touching it and break through especially hard blocks. However, he is forced to plod along at a snail's pace and can barely jump at all. Constant walking or falling into water will relieve Wario of his girth.
Flaming Wario (aka Hot Wario)
A somewhat uncontrollable transformation in 'Wario Land 2-4', Flaming Wario starts with his pants caught on fire, in which he runs back and forth across an area and can only jump. The flames eventually spread across Wario's entire body, so that he can no longer jump but can control where he's going. While on fire, Wario can light torches and destroy certain blocks. Jumping into water can cure Wario, or just waiting until he burns himself out.
Flat Wario
Often unavoidable, the falling weights in 'Wario Land 2-4' can squash Wario into a pancake, forcing him to waddle along the ground and shorten his jump. On the bright side, he can slip through passages he would normally have to crawl though and drift through the air, making it easier (or possible) to reach some hidden areas. Getting wet will return Wario to his normal shape, as will certain enemies and machines.
Frozen Wario (aka Ice Skatin' Wario)
If Wario comes into contact with the breath of a snowman, blue bear, or yeti, he is frozen solid and sent skidding backwards along the ground until he runs into a wall. Though it is usually an inconvenience, Frozen Wario can be used to slide past spikes or other obstacles.
Invisible Wario
After getting covered in a potion thrown by Mad Scienstein in 'Wario Land 3', Wario becomes invisible, even to the player. The occasional flicker and clouds of dust are the only sign of his location. While invisible, Wario can sneak past watching eyes that will close doors if he tries to pass through otherwise. Getting damaged or passing through a pipe will return Invisible Wario to normal.
Puffy Wario
Presumably proof of Wario's allergy to bee stings, Wario's face swells like a balloon when he is stung or shot by certain arrows. As Puffy Wario, Wario will float upwards uncontrollably, though able to control his horizontal movement, and only running into spikes or the ceiling will turn him back to normal. In Wario World, the Winter Windster will puff Wario up when it possesses him, though he looks more like Balloon Mario...
Snowman Wario
Wario will become Snowman Wario if a snowdrift falls on top of him. As Snowman Wario, Wario can't jump high or walk fast, but he can defeat enemies by running into them. If Snowman Wario steps onto a slope, he will roll into a ball that can break through some blocks and crush enemies. If he runs into a wall, he returns to normal. Snowman Wario still can't break through blocks when he first starts to roll; in some cases you'll need to find a way to give him more momentum first.
Vampire (Bat) Wario
Wario's most useful transformation, Vampire Wario comes to be if Wario is bitten (well, touched) by a Minicula. In 'Wario Land 3', Vampire Wario can move around and jump normally, though he destroys any enemy by touch. Pressing B will turn him into a vampire bat, which can fly in spurts. In 'Wario Land 4', Wario goes straight to bat form and cannot become the vampire. By touching a garlic clove, falling into water, or moving into a patch of light, Wario will return to normal.
Vampire Wario also appears as the final boss of 'Dr. Mario 64' (granted you didn't lose at any point prior to this match in hard mode or greater), if you chose to play as Dr. Mario, when Wario eats the Megavitamins and transforms into him. Defeating him in one battle will make Vampire Wario a selectable character.
Zombie Wario
Wario's most grotesque transformation, Zombie Wario is the result of being touched by a ghost, bitten by a zombie, or spit at by a skeleton in 'Wario Land 2-4'. As a zombie, Wario is impervious to damage, can defeat enemies by touch, and can fall through thin floors. However, he also moves slowly and can barely jump. To return to normal, Zombie Wario can fall into water or walk into a patch of light. In Wario Land 4, certain enemies can only be damaged if Wario is a zombie.