From the Super Mario Wiki
The Bone Piranha Plant is a subspecies of Piranha Plant debuting in New Super Mario Bros. 2. They are skeletal versions of their relatives with some changes, such as a grayish cranium, holes rather than spots, greenish-gray lips, vertebrae instead of a plant stalk, and dark, gray leaves that appear dead.
New Super Mario Bros. 2
Besides these graphic changes, its attack patterns appear the same than those of a regular Piranha Plant. However, unlike normal Piranha Plants, they are immune to fireballs. They can be defeated by a Super Leaf tail spin, Star or a Gold Flower, however. They are only found in Fortresses and Castles.
Mario Party: Island Tour
Bone Piranha Plants also appear in Mario Party: Island Tour on the Bowser's Peculiar Peak board. If the player lands on a space that moves them forward a number of spaces, the Bone Piranha Plant appears, chews them up, and then throws them forward.
Mario Kart 8
In Mario Kart 8, Bone Piranha Plants appear as obstacles on Bone-Dry Dunes. Here, they behave just like other Piranha Plants in the Mario Kart series, trying to bite nearby drivers.
- Interestingly, Venus Fly Traps, the plant which Piranha Plants are based off, don't have bones.