The game seems to be pretty small so there are only 3 bosses in all. The one guarding the first stage is a giant worm shaped... thing. Basically the only requirements in the design document were that it had to have round shapes that make up the head, body, and tail, so it didn't technically need to be a worm, per se. The body and head needed something to repel jump attacks (spikes were the go-to option, though I wish I'd have had more time to explore something more creative...), and the tail is the weak point which has to be hit with the gun.
Enough talk though. Here are a few ideas I drew up for it.
Very little animation required from me on this one. The segments are moved through code so it's mostly a matter of the developers piecing them together. All I really I had to do was draw up frames for the head to open and close its mouth. We'll take a look at the final result next time.