Fireworm is type of Worm that pops out of a pool of lava in the ground and spits fire at Isaac. The fire persists for a while, and can be put out by shooting it. It can only be found when there's lava in the room.
Bug! In some cases, Fireworms can spawn in rooms with pits instead of lava. This, however, is purely a visual glitch and does not affect gameplay.