The Terrorclaw is exclusive to DST introduced in Return of Them.

At low Sanity (below 10%), Terrorclaws spawn in place of Terrorbeaks and Crawling Horror for players sailing the ocean. When a player lands, all Terrorclaws following them transform into Terrorbeaks. Inversely, Terrorbeaks transform into Terrorclaws when the player boards a boat.

Terrorclaws only move in the water and teleport close to the player to attack. When attacked, they do not disappear and continue to attack the player.

Killing a Terrorclaw regenerates 15 Sanity and drops 1 Nightmare Fuel plus a 50% chance of an extra one.

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Information and picture taken from the Don't Starve Wiki.