Many criticisms have been leveled at the first Pixar to feature a female protagonist, proving it not to be as high quality as the best of the Pixars which all, incidentally, feature male protags.
But nobody's criticized the choice for Angus, Merida's equine companion, transport and Greek chorus. He's a Clydesdale! Clydesdales and their distinctive fetlock feathering did not exist until fairly late in the 18th century.*
The first trailer for Brave prompted me to wonder how such a tiny person could keep her seat astride such a wide-backed horse (in later trailers Merida appears older than she did in that first one -- in which she seemed to be 9 at the oldest). But I didn't realize the horse was a particular breed, rather than the the generic Great Big Horse.
Never forget ladies and gentlemen: movies exist to tell you lies!
* From wiki: The Clydesdale takes its name from Clydesdale, the old name for Lanarkshire noted for the River Clyde. In the mid-18th century, Flemish stallions were imported to Scotland and bred to local mares, resulting in foals that were larger than the existing local stock.