Having recently received my copy of The Gathering: Reuniting Pioneering Artist of Magic, I read the history behind this card's art. Apparently, Melissa Benson was just told "draw a nightmare", so she decided to make it into a pun on the word "mare". Somehow, knowing that this card's art was probably not what Jesper Myfors (then art director) was looking for makes this card that much more cool.
As for the card itself, it's pretty decent. Black doesn't really hurt for nice flyers, but this is pretty good in mono-black decks. It's probably going to be a 6/6 flyer for 6 and it'll only get bigger with every land drop. The nice thing about cards like this is that they can be easily scaled down because their power and toughness will be at the same ratio for mono-colour decks. I kind of would like to see this at around 4 CMC, but 6 is fine for windmill slamming in Limited.
Pros: Scales with land drops, flying
Cons: Costs 6