https://www.youtube.com/watch?v=R8U5tnnCvQI
I don't really delve too much into theory making but I find it very odd that people are ignoring such a glaringly obvious hint that's very easily accessible in the files.
Like, a variable named Toriel hiding the identity of one enemy that gets killed without any player input, that also happens to be a representation of a dark coloured deer. The fuck is she hiding from everyone.