Okay, this is just because I really wanted to doodle and sometimes think too much about the designs of games I'm never going to actually make. I could have expanded this thing downard and drawn even more forms, but I was starting to get tired of it.

BASIC MOUSE
* The default form in the game, unlocks double jump at the end of the intro stage and thus allows you to reach the areas where you start unlocking the others.
* Boomerang found about halfway through the intro stage. It can potentially hit an enemy twice and grabs collectibles. Only one of it can be onscreen at once though, and it's on the slow side so far as projectiles go.
* Overall, this form is generally the worst but the double jump can really come in handy during precision platforming segments.

CYBORG MOUSE
* Combination of basic mouse with a wind-up mouse.
* Lemons Gun fires projectiles that reach a full screen's length and can have up to three onscreen at once. They do the same damage as the boomerang.
* Lemons Gun can be upgraded to be a charged weapon, where it becomes a much larger piercing attack if the fire button is held down for a full second before release and the player avoids getting hit.
* Slide Kick gives invulnerability frames and causes minor damage, and against certain larger enemies can trip them up.
* Slide Kick's main purpose, however, is allowing you to get through passages with low ceilings that give the other forms trouble.

SHOTOCLONE MOUSE
* Punch can break bricks, wooden boards, and cars as well as having better damage (and worse range) than almost all other attacks.
* Special attacks are triggered basically how you'd expect; rotating the directional controls in specific ways before hitting attack (or jump in Tornado Kick's case).
* Throw Punch is a slow-moving projectile with a big hitbox and does as much damage as the punch. Only one can be onscreen at once.
* Jumpercut is the highest vertical jump in its own right, easily rivaling the double jump. It can hit a button on the ceiling.
* Jumpercut also has the same damage as a punch, and against some enemies pulls them into the air until their invulnerability frames expire, causing them to be damaged by it twice.
* Tornado Kick is a pretty good horizontal jump, allowing the player to pass through certain passages that have both ceiling and floor hazards.
* Oh and I guess Tornado Kick is also an attack. Same damage as the normal attacks on the other forms, but it can hit enemies with short invulnerability periods several times at close range.

EXPLORER MOUSE
* A snazzy dresser who knows how to reference one of the best movie series and multiple videogames all at once.
* Whip can strike in five directions: Left, up-left, up, up-right, and right. It doesn't do down. However, it has a notable delay and short reach.
* Whip can grab projectiles, just like the boomerang.
* When upgraded, the whip can grab fixtures anchored to the ceiling and allow a swing.
* Once found in the ruins, the Ice Sickle allows eight-directional aimed melee attacks with higher speed and lower reach than the whip.
* Ice Sickle can freeze some enemies into platforms, extinguish fires, and does extra damage to some bosses.
* Ice Sickle adds an edge grab (or perhaps just makes an existing edge grab more generous)
* Wall Kick, once unlocked, has its expected effect on exploration.

Other forms here.