Yoshi comes second when you battle against everyone. There are eighteen yoshis, each very weak--use aerial attacks especially.

Yoshi's field is quite basic(but small[bad for Yoshi!!!]), and perfect to sneak in Pikachu's Thunder on above opponents. In Vs. Mode, there are two clouds on the right and a cloud on the left. Items will fall here, and you can also use this to get away from everyone. Don't stand on them too long, though, or they will disappear!

Yoshi
When you play as Yoshi, big playing fields will let you rack up major damage without falling off while throwing others out of the arena with your strong attacks.
A+Up(Smash): Headbutt
Headbutt streches all around Yoshi, and is pretty strong, too. If your opponent is behind you, you'll send it backwards. If they're in front, you'll knock it into the sky.
Anti: Shield very quickly.

A+Up(Normal): Head Thrash
Use this to knock away crowds.
Anti: Jump away when you see others get hit.

A+Side(Smash): Super Headbutt
Just like Link's.
Anti: Shield!!!

A+Side(Normal): Mid-Kick
Use this to keep an opponent off the edge.
Anti: ???

A+Down(Smash): Double Tail Trip
Yoshi hits foes on both sides far away.
Anti: Shield.

A+Down(Normal): Tail Trip
Slightly more effective to keep an opponent off the edge than the Mid-Kick.
Anti: Come up from under it.

A: Kick
A Twice: Double Kick
A+Dash: Head Charge
Use this to get back into the brawl.
Anti: Jump up to avoid Yoshi's Head Charge.

Mid-air: A: Karate Kick
Just like Mario's Mid-air A attack.
Anti: Counter with a stronger attack.

Mid-air: A+Forward: Somersault Pulverize
This attack shoves enemies down like Samus' Down Punch.
Anti: Try to keep agile.

Mid-air: A+Back: Back Karate Kick
Stronger than the Karate Kick.
Anti: Go above or under.

Mid-Air: A+Up: Flip
This has no real use.
Anti: Attack down. Mid-air: A+Down: Drill Stomp

An alternative to the Sumersault Pulverize.
Anti: Double- or triple-jump back on RIGHT after.

B: Egg
Once in an egg, you can easily rack up damage on your foes.
Anti: Jump.

B+Up: Egg Throw
Throw these on your way back into the arena. Don't forget to control it with the analog control stick.
Anti: Shield or throw som'in'.

B+Down: Hip Drop
After baraging everyone with eggs, slam them down with this.
Anti: Shield.

R: Throw
Same as Link.
Anti: Hop up.