Prickly Goomba

Prickly Goombas are Goombas that hide inside spiked chestnut shells that appear in New Super Mario Bros. Wii, New Super Mario Bros. U, New Super Luigi U and New Super Mario Bros. U Deluxe.

New Super Mario Bros. Wii
Prickly Goombas first appeared in New Super Mario Bros. Wii, where they are only encountered in World 9-7. The players will sustain damage if they jump on their spiked shells. Prickly Goombas will remain stationary until their shells are burned open with a single fireball, thus reverting them to a regular Goomba. They can also be frozen by ice balls.

New Super Mario Bros. U / New Super Luigi U
Prickly Goombas reappear in New Super Mario Bros. U. In this game, they appear only in the level Prickly Goombas! in Frosted Glacier. They retain the same behavior from their previous appearance.

In New Super Luigi U, Prickly Goombas are encountered in Fire and Ice.

Trivia

 * Prickly Goombas are quite similar to Pestnuts from Mario & Luigi: Superstar Saga, in that they are both chestnut-themed enemies that hold a certain enemy within them (Goombas and Beanies respectively) that act their usual manner when released.