Balrog is a toaster. As shown here by this short CG animation from Cass Carpendale. The 3DS eShop version of Cave Story releases this week, in case you forgot.
Our own JC Fletcher has played the game and posted his impressions, calling it "the best way to experience classic Cave Story in a handheld setting."
Buy: Nintendo 3DS and 3DS XL consoles See also: More Cave Story news and media