When you first enter the Ice Planet, you need to jump from iceberg to iceberg over freezing cold water. Metal Piranhas will pop up out of the water and will occasionally hit the player. Most of the time, when a player gets hit, they will fall into the lake and will freeze, causing them to respawn at the beginning of the area.
To get past them, don't try to jump over it, and keep your balance on the iceberg. The Metal Piranhas will eventually leap over you. Then jump to the next iceberg.
The Piranha will sometimes actually help the player. If the player is above the water and between two icebergs, the piranha will sometimes knock the player back onto the iceberg.
The Piranha resembles a knight's helmet with a tail and a fin on top.
It is possible that the Metal Piranha is made of stainless steel as it never rusts, even though it lives in the water.