Community-reported
Bowhead Whale
A reported Island 5 boss, distinct from the (verified) Mutated Bowhead Whale. Defeating it reportedly produces the bait needed for the final boss.
How to Find It
Summoned with a Fish Bucket, per Destructoid.
Rewards
Reportedly used as bait to summon the Mutated Bowhead Whale after being defeated.
Game8 lists both "Bowhead Whale" and "Mutated Bowhead Whale" as separate bosses; Destructoid's boss table independently does the same and adds the bait chain (Fish Bucket → Bowhead Whale → [defeated] Bowhead Whale → Mutated Bowhead Whale), which is a coherent enough mechanic to treat this as a real distinct encounter rather than a listing error, though it's still only Destructoid's specifics backing that chain.
Related
Mutated Bowhead Whale
The final boss. Named identically across every source checked, and the fight itself is independently confirmed by an official Steam achievement.
Volcano
The final island area, home to Goblin Shark, the Bowhead Whale, and the Mutated Bowhead Whale final boss.
Fish Bucket
Reported boss bait for the (non-mutated) Bowhead Whale on Island 5.