From the Super Mario Wiki
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 Morty Moles as they move faster after being hit the first time.
- ^ Nintendo Power New Super Mario Bros. guide, p. 11