The Crusader Bible 

Digitized and online at the The Morgan Library & Museum this has a collection of various paintings much of which include horses and tack. Here is the description from the site itself:

"Old Testament Miniatures with Latin, Persian, and Judeo-Persian inscriptions

France, Paris, 1240s

The Crusader Bible, also known as the Morgan Picture Bible, the Maciejowski Bible, and the Shah ‘Abbas Bible, is not only one of the greatest medieval manuscripts in the Morgan, it also ranks as one of the incomparable achievements of French Gothic illumination.

The miniatures represent one of the greatest visualizations of Old Testament events ever made. Some of the stories and their heroes are well known, but there are also accounts of less familiar Israelites who fought for the Promised Land—tales that resonate to this day. There are incredibly violent battle scenes in which the implements of war are so accurately depicted they could be replicated. And there are scenes of everyday life, love, hate, and envy, as well as adultery, rape, and murder—all set in thirteenth-century France." 

Going through the images, some are very highly detailed, and the others are not done well at all. This implies that it was a collaborative work, and at least one of the illumination artists did not care to show as much detail as the others. There are still plenty of images to go through.