It's just a rat. A, er, rather big . . . semi poisonous rat.

[He gives her a little nonverbal welp and a shrug.]

They were released on the ship. I got bit quite a few times, and most of us got sick, but, er, Sirius kept one. Brunhilda, he calls her, heaven only knows why. But we've both put spells on the cage, so she won't get it out.