I fail to see how a Nightmare is a dog, since the tibia creature page tells us:
"One thing that always eludes the demons and their powers are dreams. The gift of dreaming is only granted to mortals. Still some demons lurk at the borders of dreams, watching, drooling with hatred and greed. Sometimes a dreamer gets some distant feeling of those unwelcome spectators. Such dreams often turn into nightmares."
Later on it goes on to say:
"heir physical form reflects their constant travel to a place they do not belong to and takes the shape of a ghostly horse creature."
So I can understand it being called a Demon, a Dream or even a Horselike creature but forsure it's no dog.