It is a challenge to make an already anthropomorphized character into a Were but gave enough to Lungs here to make it count.

Archibald Fellows was Amos’ first mate, but the whole crew only ever called him Lungs after the first time they visited his home port and dropped in on his mother. Mrs Fellows believed an old wife's tale (Being an old wife herself) that eating lung would help Archie hold his breath longer if he was at sea. Force fed the whole crew lung sandwiches while they were there the nickname stuck even after an attack left Lungs cursed.
On the full moon, Lungs found his muscles, fur and claws extra thick, he would double in size and his bones would snap and reform with digitigraded legs and an arched back. As you can imagine, that left the crew in a bit of a bind when the beast took over while they were all stuck on a boat together... It got to the point, Lungs wanted them to leave him on an deserted island with one shot in a pistol. But Amos wouldn't let a friend go like that: "We die in a blaze of glory, Lad, or we nay gonna die at all!"
So they sort out a kat who promised a cure and found one... Well they found a snake oil salesmen who said it would solve their problem of being locked in the hull one night a month ¬¬ Taking the elixir reversed his forms! So now he is stuck in the monster's body all month apart from on the full moon where a much smaller and weaker kat is snarling and biting at his friends like a mad kat... At least it's easier to lock him in the brig till morning this way.