A Porcupine is an obstacle that can be found on the track and ledges of Yoshi Valley. Running over one causes you to spin out, which can result in violently falling down the cliff. Depending on what section of the map you choose to take, (because it's just so dang confusin') areas can be so densely packed with Porcupines that either master hopping/drifing skills or extreme luck are needed to pass safely. Other places are only dotted with them, but they're still deadly.
On the Wii U/Switch remake of Yoshi Valley, Porcupines have been replaced by Goombas, which are a whole lot easier to dodge, in part because of the newer controls, and just the overall number of them is decreased.