Splunkin

not to be confused with Jack-O-Goomba, they look similar

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, killing it and earning points. The Splunkin is a relatively small and harmless enemy, though they are sometimes found in larger groups.

Since Crowber, a raven enemy, is usually flying around, Mario may try to dodge the bird and end up getting hit by a Splunkin. Jack O' Goombas highly resemble Splunkins. Splunkins behave just like Rexes (and in Super Mario 3D Land, Morty Moles) as they move faster after being hit the first time.