A little robot on wheels that, once Mario gets close enough, stops moving and starts a fuse.
When the time is up, it explodes, but before doing so, it'll eject a small copy of itself out of its helmet, that grows to full size once it lands and repeats the cycle (until Mario cruelly stomps it).
Fuse timers are adjustable, meaning it can get quite dangerous and much harder for the player to stomp if you set its fuse lower, while longer fuses make it a lot easier to navigate around.
Graphics are included, they use up the unused tiles in the Spiny/Wiggler GFX file, but there are defines to easily move its graphics (and palettes) around.
Moderator's note: This thing uses an obscene amount of tiles for its size (you thought a 16x16 sprite would need but a single tile? joke's on you, this lil' bugger uses 7). Because of this it is highly recommended you also patch NMSTL if you want to use this sprite, otherwise sprites will occasionally turn invisible.