Big Urchin

A Mega Urchin is a colossal sub-species of Urchin that first appears in the game New Super Mario Bros. Wii.

New Super Mario Bros. Wii
Mega Urchins only appear in World 4, and just like regular Urchins, they will float up and down, trying to poke Mario and the others. However, unlike regular Urchins, they are almost invincible, only vulnerable to a Star. Also unlike regular Urchins, their pupils follow Mario's movements. They mostly appear alongside regular Urchins.

New Super Mario Bros. U
They appear in one of the Sparkling Waters levels, Urchin Shoals, retaining their behavior from New Super Mario Bros. Wii.