Giant Piranha
The second boss, unlocked after clearing Spidercrab.
How to Find It
A gameplay video independently confirms a leech-collection quest is required for this island's bait, consistent with, but not identical to, the leech step this boss was already tied to.
Boss health bar reads "Giant Piranha" verbatim in the official gameplay trailer, matching all three guide-site sources exactly. One source ties this boss to a leech-collection quest first; the others don't mention that step, so treat the leech requirement as unconfirmed.
Related
Boss Fights & Island Progression
Islands unlock sequentially: defeat the boss guarding your current island to progress to the next, more dangerous one.
Spidercrab
The first boss, guarding the starting island.
Forest
The second island, reached by boat rudder. Home to a leech-collection quest, Giant Piranha, Sunfish, and The Old Pike.