Realize that since Spirit Tracks hasn't arrived yet, the following content is speculation based on videos shown to the public. When the game gets released, we'll be sure to update the content.
Large Beetle - Armored Insect
We believe this to be the second boss in the game (as the demo videos show five hearts which means it's either going to be the second, or third boss).
Phase 1: Clear purple mist, attack abdomin
Phase 2: Blow spikey larva at boss