They usually site things like brain capacity, development and structure.
They say that dreaming is a human condition, that what appears to be dreaming is merely a projection.
They base their findings on anatomical studies, lab work and theory.
Watch Tessa's ears, eyes, nose, body and tell me she does not dream.
I don't know what she dreams because I am human and I cannot know a horse's dreams as she cannot know mine, but I know she has them.