Those of you familiar with piranha plant physiology will note that he looks rather young. That's because when you encounter him he's peacefully incubating in his egg. At this point you and your violent plumber avatar slam into the egg at an incredible speed, cracking its delicate exterior.
If Peach wants a space omelet, she gets a damn space omelet.
With the shell still partially intact, the poor newborn stands upright and begins wandering around with his tail trailing behind him.
"Ah, the miracle of life. Well, time to beat the shit out of it."
Blind, aimless, and undoubtedly terrified of the sudden harsh reality that's just been thrust upon him, Dino gets his second trouncing when Mario knocks his tail into the sky. The elastic appendage comes careening back into the egg, shattering the only remaining barrier between the baby and the horrible outside world that, as far as he's concerned, means only to kill him in a painful and humiliating manner.