Claw Rat
Rats exist across Creation, and this is certainly true across the Western Seas, where they hide in holds to chew away at vital supplies. But the sailors of Western ships dread another pest even more than the rat: the Claw Rat.
The Claw Rat is in many ways a rat, but one which is hairless save its whiskers and bears a thick chitinous shell and two additional arms ending in sharp pincers. While these mean the creature has more difficulty than its rat kin in squeezing through small spaces or moving with speed on land, its comfort in the ocean means swarms of the things can sometimes assail ships by clambering up hulls, and its claws mean it can penetrate even secured cargoes. Those claws and its shell also make it more resistant to attacks and traps which might deal with regular rats.
A small advantage to sailors on long voyages is that claw rat meat possesses a flavour reminiscent of crab itself, a not-unpleasant supplement to the diet.