Warning: Answers may contain (hopefully hidden) spoilers from the end of the series.
Kiroumaru is a leader of the bakenezumi (monster rats or queerats) but he looks a lot different from all the other ones in the show.
On the left is Yakomaru, who looks like pretty much every other bakenezumi in the show. On the right is Kiroumaru who is quite different from Yakomaru. In a few of the episodes, I think I heard characters refer to Kiroumaru as a bakenezumi, and nobody questioned why he looks so different. Is he really a bakenezumi, or is he some other species that joined a bakenezumi tribe?