Dino Piranha is the first boss encountered in Super Mario Galaxy.
To defeat Dino Piranha, Mario or Luigi (depending on which mode you were playing in) must hit its tail with a spin which would cause the tail to backlash and hit the back of its head which would then cause it to lose some petals which would then make it enraged. When enraged, the Dino Piranha will be faster. When enough damage is dealt, the Dino Piranha will explode.
Fiery Dino Piranha
Present in the Melty Molten Galaxy, a different version of Dino Piranha appears as the boss of the area. The Fiery Dino Piranha fights exactly the same as the regular Dino Piranha, with some changes to give its fiery approach: the tail occasionally bursts into flames, making it precise when to hit the tail to avoid being burnt. After the first hit, the Fiery Dino Piranha leaves behind a trail of fire from its footsteps, and then occasionally shoots out a fireball from its mouth after the second hit. After the final hit, the Fiery Dino Piranha explodes.