From the Super Mario Wiki
Harisenbon (ハリセンボン, lit. "porcupinefish") appears only in Super Mario Land 2: 6 Golden Coins. It is a fairly rare enemy and appears only in the Turtle Zone. It usually appears flat and slender, almost eel-like, but it regularly inflates itself into a spiked ball. When inflated it can only rise upwards, so it moves towards Mario when deflated and attempts to block his path by puffing up and getting in his way. They can be defeated with Fireballs.