From the Super Mario Wiki
This article is about the enemy. For information about the similarly named Rock-Candy Mines
course, see here
Stalking Piranha Plants
in an Enemy Course
in New Super Mario Bros. Wii
Stalking Piranha Plants are sub-species of Piranha Plants that first appear in New Super Mario Bros. Wii. They appear in World 5 as common enemies and World 6 as rare enemies. When Mario isn't around, they walk around on two legs, like Ptooies. When Mario approaches, however, they stretch out their necks to block Mario's path and to attack him. Stalking Piranha Plants also appear as the enemy of the World 5 Enemy Course. These enemies can be defeated by a fireball, being frozen, hit with a Starman, or being hit by a shell. Like most Piranhas, these enemies cannot be picked up when frozen.
Stalking Piranha Plants return in New Super Mario Bros. U in the Soda Jungle and the Rock-Candy Mines, with the same tactics.