I'm going to go on a bit of a rant here: I've kind of got a bit of a bias against Yoshi, both as a character and as a fighter.
In the actual game, whenever I go against him, I really don't like how his drifty-ness always catches me off guard (not like I can talk, though), and when I play him I always end up screwing up my jumps whenever I go offstage, resulting in an SD almost all the time.
But you didn't come here to hear me throw a hissy fit about the game. Story-wise, Yoshi doesn't have much going for him either. I mean, I haven't played a Yoshi game (you can hold that against me), but from what I know he just helped Mario and Luigi get delivered to their parents when they were babies and now he's Mario's best friend. It's fine for a supporting character, but it doesn't give me much to work with as a main character. Anyway, I digress. I really tried to do the best I could with what little I had.
A monstrous grumble shook the forest floor. Leaves trembled, and the flow of wind over the grass parted for the new force that came crashing down on the peaceful woods. Birds scattered in panic. A tree fell to the dirt below with a resounding crack.
Plowing through the forest, a line of bright yellow vehicles tore through the dense growth, the Koopas sitting at the helms uncaring where the towering trees they brought down ended up, so long as they fell in the first place. And at the head of it all stood Wendy Koopa, one of Bowser's many children.
The crashing and roaring slowly awoke the dinosaur-like creatures from their sleep beneath the shrubs on the forest floor. The Yoshis looked around at first, completely unaware of what was going on. Having the trees fall down around them did a good job to scatter them deeper into the woods.
Above it all, Wendy cackled. The forest fell to the dirt below, and troops of the turtle-like Koopas rushed in after the line of machinery to drag the trees away. Wood crackled and popped. Nothing could stand in their way.
Then, the ground split. It was just a small crack, at first. A tiny crevice in the earth between two shrubs. As the bulldozers closed in, the cracks grew larger and began to make noise against the roaring engines. In a shower of rocks and dirt, the forest floor opened up its jaws, and one of the bulldozers sunk beneath the grass.
One by one, the construction machinery around her began to crumble. Bits of the earth splattered against the sides of the yellow paint as the ground sucked them up, and as a panicked clamor rose from the Koopa Troopas, Yoshis began to emerge from behind the twisted metal and smoke, pressed against the ground and shaped in the likeness of a tank. More of these "Mole Tanks" popped out of the ground, and the Koopas gradually organized their ranks.
A brawl started to form in between the pieces of broken machinery. Some punches were exchanged, some eggs were thrown, but the Koopas found themselves struggling to match their opposition. Wendy watched everything unfold, and found herself worrying.
As luck would have it, more Yoshis tumbled out of the forest. A great cloud of dust wafted into the air, clinging on to their every step. There were countless Yoshis of all different colors among the stampede that had formed; the individuals blended in with each other like a shuffling, shimmering rainbow. When the Koopas caught sight of the endless torrent of dinosaurs slipping through the trees in the forest, they fled.
Reluctantly, Wendy followed her Koopa troop away from the island. She took one last glance at the Yoshis closing in, and as she ran, she muttered, "Daddy's not going to be happy with this."
A great cheer rose from the dinosaurs as they watched the intruders flee from the devastated forest. Sure, a few trees had been knocked down-it would take years for the vegetation to repair the damage done-but in the end, much of their home had been left untouched. They were simple creatures and didn't have much in the way of extravagant celebration, aside from the excited chattering that rose into the air. A few jumped cheerfully with each other.
Amidst the celebration, one of the Yoshis, a green one, glanced around warily. He was the leader of this particular group, a bit more cautious than the rest. He had also gone on adventures with a certain red-capped plumber before, and he was worried that there might be more Koopas hiding around.
Slowly, Yoshi snuck through the crowd of other Yoshis, making his way to a particularly high rock. He fluttered his way up to the top and looked around. The vast blue sky stretched out above him as he scanned through the open landscape. He didn't find a trace of the Koopas lying around, save for the broken and battered machinery left over from their attack on his forest. What he did find was a spaceship, almost a speck against the cloudless sky, drifting through a light breeze passing by.
"Mario's buddy is full of vim and vigor and can't wait to get into the ring to show the other contestants that this little dinosaur's fighting skills are far from extinct. Yoshi is one of the fastest fighters, and has an impressive jumping range. His egg attacks are a little weak, but they give him an advantage at a distance." ~ Super Smash Bros site on Yoshi