Board Thread:False Info and Speculative Discussion/@comment-24.130.28.230-20150721180826/@comment-26879545-20160201235244

IIRC, Meredith hinted that Banshees were a class of "monsters."

When she and Lydia were talking, Lydia says "Not all monsters do monstrous things." While Lydia was referring to Scott, Meredith says something like "Like you," to her. Meredith knows Lydia's a Banshee. I don't remember if we found out Meredith was the Benefactor by this conversation, but as a Banshee herself, she was the one acting "monstrous" out of the two of them.

As of now, the Supernatural is consisted of monsters, but not all monsters are supernatural. I think that's what Paul's trying to say. Lydia may be a type of monster, but as defined by the show, no Supernatural creature can cross/use mountain ash. The fact that she does both means that, unless Banshees are stated in the show, or by Jeff Davis in interviews, Lydia is a monster, but she's not Supernatural.