Italian artist Pisanello left us a number of drawings showing horses, mostly heads, and elements of horse tack, mostly bridles with curb-bits.
Among them there is a collection of horse heads and horses with slit nostrils, ancient practice (started during the Bronze Age, eg Tell el Amarna relief et XVIII Dynasty other chariot horses) hopefully abandoned worldwide.
''..the Hussars and Hungarians slit their [horses] nostrils, with a view, it is said, to mend their wind, and, at the same time, to prevent their neighing in the field; it being affirmed that horses, whose nostrils have been flit, cannot neigh. It has not indeed been in my power to examine this particular, but it seems natural to think, that the operation can only weaken their neighing.
As late as 2nd half of the XIX century Icelanders practiced this 'technique,' in North Africa as late as the XX century, and in Mongolia well into 1920s in order to heal broken-winded horses, but allegedly well into the 1970s in Iran donkeys had their nostrils slit.
William Youatt, The Horse, 1870, wrote:
''The nostrils should not only be large, but the membranous substance which covers the entrance into the nose should be thin and elastic, that it may more readily yield when the necessity of the animal requires a greater supply of air, and afterwards return to its natural dimensions.
Therefore, nature, which adapts the animal to his situation and use, has given to the cart-horse, that is seldom blown, a confined nostril, and surrounded by much cellular substance, and a thick skin;
and to the horse of more breeding, whose use consists in his speed and his continuance, a wider nostril, and one much more flexible.
The inhabitants of some countries were accustomed to slit the nostrils of their horses that they might be less distressed in the severe and longcontinued exertion of their speed.
The Icelanders do so to the present day.
There is no necessity for this, for nature has made ample provision for all the ordinary and even extraordinary exertion we can require from the horse. Some very powerful muscles proceed from different parts of the face to the neighbourhood of the nostrils, in order to draw them back and dilate them.''
|I did a drawing of this horse tack|
Perhaps more information on the equid nostrils slitting in this article (I have not read it, the prof. Mary Littauer was very knowledgeable on the subject of ancient practices and horsemanship)