Spidercrab
The first boss, guarding the starting island.
How to Find It
Reached by completing the lighthouse keeper's quest (bringing an empty beer can as bait).
Frame-by-frame inspection of the official release trailer shows the boss health bar reading "Spidercrab" as one word, correcting the three secondary guide sites, which all wrote it as two words ("Spider Crab"). The in-game HUD spelling is treated as authoritative here.