Frisians?!

The draft horse was invented not to pull heavy wagons or ploughs, but to carry a fully armored knight into battle and not tire during the fight. Thus you'd see Percherons or Belgians or Frisians under their knights during the eras of mail and plate. But knights had gave up on plate when it couldn't stop a bullet and they then went light, with cloth, and that meant their horses could be bred for speed.

England saw horses imported bearing names like "barb" and "turk" and "Arabian", sleek, light, and fast, and the Thoroughbred breed was founded.

Mr Darcy never would have sat on a thick, heavy-set, large-hoofed mount, no matter how deliciously thick its mane and forelock were. He would have ridden a Thoroughbred. Everyone rode Thoroughbreds. The mane and forelock were often trimmed anyway, and the thick tails would have caught in the traces and trappings of a carriage; thus the tails would have been docked to 8" at most. And the skinny little legs of the thoroughbred, if they ever had extra hair on them, would have been shaved away.

Hollywood has fallen in love with the Frisian breed. They are quite beautiful. They're all dark, therefore easy to match. They have these lush manes and tails and their lower legs are covered with a great deal of hair. Yet while they are large and built very powerfully, they are still lighter than the draft breeds we usually see, which makes it easy for Hollywood to pretend that they're not. And that's why they're putting these black and near-black beauties everywhere they can. But they're completely out of place. Lord Nelson's coachman never would have hitched Frisians to his carriage. Mr Darcy wouldn't have had a Frisian to ride around the grounds of Pemberly.

Put Mr. Darcy on the only breed he would ever have ridden--the Thoroughbred. And to pull his coach--more Thoroughbreds, most likely "matched greys" which were pretty popular.