Under Lucario's Rule!

Petey Pirhana:
Game Series- Super Smash Bros.
First Appearance- Super Mario Sunshine (2002)
You fight Petey Pirhana on a small platform with Petey holding two cages with their own health gauges. Petey will attack and the cages occasionally slam down to cause damage.

Ridley:
Game Series- Metroid
First Appearance- Metroid (1987)
This flying dragon appears on a long platform in some base. Ridley uses flying attacks

Duan:
Game Series- Smash Bros.
First Appearance- Super Smash Bros Brawl (2008)
Runs on wheels, but sometimes "doesn't act like it". It does some physical attacks, which is why I say that.

Rayquaza:
Game series- Pokemon
First Appearance- Pokemon Ruby and Saphire (2002)
You can't be knocked out of the ring when you fight Rayquaza because you're in a jungle area. Rayquaza has many attacks that can send you flying upward.

Master Hand:
Game Series- Smash Bros.
First Appearance- Super Smash Bros. (1999)
A classic boss even from the other Super Smash Bros. game as you should know from above. Master Hand will take the position of right stage, and has numerous attacks. Its all about how his hand moves, that's how you know what attack Master Hand does.

Galleom:
Game Series- Smash Bros.
First Appearance- Super Smash Bros. Brawl (2008)
A huge robot thing... His attacks cause great damage to the opponent character.

Meta-Ridley:
Game Series- Metroid
First Appearance- Metroid Prime (2003)
Just like Ridley from before (well, he was to me). The only difference is that Meta-Ridley is constantly in flight.

Crazy Hand:
Game Series- Smash Bros.
First Appearance- Super Smash Bros. Melee (2001)
Just like Master Hand except now he's on the left side of the stage and he attacks constantly.

Porky:
Game Series- Earthbound
First Appearance- Mother 3 (2006)
Huge Spider machine, basically. Porky will whip his legs at you wildly.

Tabuu:
Game Series- Smash Bros.
First Appearance- Super Smash Bros. Brawl (2008)
The final boss in the Subspace Emissary. It has plenty of attacks.