A Splunkin is a jack-o-lantern enemy in New Super Mario Bros. They walk left and right in World 8 courses and some of the earlier Ghost Houses. If Mario bops a Splunkin once, it will crack a bit and will increase its speed. Another bop will make it explode, defeating it and earning points. The Splunkin is a relatively small and harmless enemy, though they are sometimes found in larger groups. Their name is pun to "Spelunking".
Several Splunkins in the World 3 Ghost House
Jack O' Goombas highly resemble Splunkins. Splunkins behave just like Rexes, Wigglers (and in Super Mario 3D Land, Morty Moles) as they move faster after being hit the first time.