This isn't my theory, but I agree with it, so I'm posting it.

Though it might seem a bit all over the place.

So throughout the seasons Melissa Hastings has been shady, not only is she shady, but she tends to disappear for long amounts of time.

The theory here is that Melissa Hastings made up Bethany Young as like a decoy type thing, and that would explain why when Mrs. DiLaurentis took her to the stables, she was using the riding hat that had MH written in it.

The reason that she would want a fake name is because her family is all about reputation, she says she pushed Bethany into the hole, but what if she is just a psycho? Also, Bethany's voice kind of sounds like Melissa Hastings, and the only person we know Mrs. DiLaurentis having an affair with was Melissa's dad.