Thread:Vizard6991/@comment-29727085-20161006004302

In arc 6 she says

「メィリィの場合、そういう角の有無とは無関係に影響する. 理屈はイマイチ、説明されてもよくわからねぇんだが」

So she can control horned or hornless demon beasts. It was mentioned in arc 6 when they went to her cell.  