The word nightmare is not a reference to a mare female horse seen at night but to a terrifying dream one in which the dreamer experiences helplessness anxiety fear and possibly sorrow. The symbolism is that of a horse trotting through your dream in some uncontrollable fashion wreaking all kinds of havoc.
Let me advise you my dear young friend-- nay let me warn you with all seriousness that should you leave these rooms you will not by any chance go to sleep in any other part of the castle. It is old and has many memories and there are bad dreams for those who sleep unwisely.
Grandma sees these things when she talks about them and gestures with her hands like shes painting brush strokes in the air. The way Grandma paints her dreams for me theres a low sky.