Slime hails from the Nintendo RPG known almost as much as pokemon, Dragon Quest, as it's mascot. Slime has been rumored before, and it seems like a good choice. Slime is a common enemy in Story Mode, and beating Story Mode unlocks a playable Slime for Classic Mode.
B. Splash: Slime splashes itself against the ground to damage enemies and slow them down.
<-B-> Slide: Slime turns into a puddle and slides, causing opponents to slip on it and take damage as a result.
B^ Slime Stack: Two slimes get beneath slime and toss him upwards.
B V Bubble Slime: Slime melts and sends bursting bubbles outwards.
Final Smash: King Slime
Seven other slimes merge with Slime, creating King Slime, and the rest is like Giga Bowser, only with cooler attacks.