Semi-aquatic rodent (5)
(Other definitions for coypu that I've seen before include "South American rodent bred for its fur, nutria" , "Large South American aquatic rodent" , "Large semiaquatic rodent" , "aquatic mammal" , "Creature in fur coat" .)
(Other definitions for water rat that I've seen before include "Large semi-aquatic vole" , "Swimming rodent" , "beastly thing" , "swimmer" , "One employed by the bank" .)