From the Super Mario Wiki
Chain-Chomp Chase is a minigame in Wario's Carnival in Dance Dance Revolution: Mario Mix. A giant Chain-Chomp starts chasing Mario or Luigi. The player has to quickly tap left and right alternately on the Dance Pad to run fast enough to escape the Chain-Chomp.
The Chain-Chomp came out of nowhere! You'd better pick up the pace or he'll catch you!