Hello. A friend sent me this picture with a boss which Reiner transformed in,in map1. He also said that there are many unused mobs.In fact,i suggest adding this boss/ a boss in Deungsan Peaks as a new boss with darkness resistence
The monsters which are not used are either reserved for future updates or bugged in a way we cannot resolve. The only reason the latter types are kept is because they can be reused in other forms, other than mobs.
This one, you talk about, has broken animations and we will not give it the use you propose.