So my understanding of cavalry in Europe is that from the Medieval period to the modern period there was a transition from heavy warhorses to lighter cavalry due to technological advances. Now light cavalry had always existed, especially in the Roman Empire where they had dedicated horse breeding centres.
Were these modern period horses better from a physical perspective or simply more modernly equipped/trained? I would suppose it comes down to the definition of better, in the context of light cavalry I'd imagine the criteria are: greater endurance for given size, greater speed for given size, more intelligent/ more easily taught and less easily frightened.
Another way to phrase this would be, if two horses looked exactly the same and had the same size and shape except one was from late antiquity whilst the other was from the modern period, would there be a difference? Had selective breeding in the modern period produced a horse that performed better given the same physical characteristics, or did it simply produce a horse with more of those preferable characteristics that a horse in antiquity could potentially possess as well?
if two horses looked exactly the same and had the same size and shape except one was from late antiquity whilst the other was from the modern period, would there be a difference?
Well... they couldn't look exactly the same and have the same shape because there are dramatic differences between horses then and horses in the modern period. It's like asking, "If a '21 Ford Model T and an '06 Honda Civil looked the same and had the same engine, would there be a difference?" The question contradicts itself.
Like with most things, when it comes to horses, form follows function. A big horse with large muscles will be stronger. A tall horse with long legs and conditioned muscles will be faster. A smaller and daintier horse will be more agile. If two horses are exactly the same size and shape, they would be exactly the same type of horse and thus have no difference.
But. Horses were not the same size and shape.
So my understanding of cavalry in Europe is that from the Medieval period to the modern period there was a transition from heavy warhorses to lighter cavalry due to technological advances.
Eh... Kind of. There never really was a "heavy" warhorse. There were bigger and more powerful warhorses, but big, bulky, drafty horses (like the Clydesdale type horse) were never the ideal war horse. It would be like going to war on a tractor. Sure, it has a lot of power, but it's poorly optimized for the task. A big, heavy horse is slow, too wide to ride comfortably, and lacks the agility and maneuverability of proper war horse.
The ideal war horse in the medieval period was called a Destrier, which is really a precursor to the modern and contemporary "Warmblood."
Quick aside- there are three general "classes" of horses- Hot blood, cold blood, and warmblood. A hot blood is fast, has a high strung temper, and is typically smaller. Think Arabian horse, Barb horse, or Thoroughbred. A cold blood is a draft or pulling horse, like a Clydesdale, a Shire, or a Percheron. They are usually big, strong, slow, and easy going/mellow. A warmblood is, in theory, a cross between the two. Most sport horses are now warmbloods.
The destrier/warhorse became more refined as people began to apply more methodology to breeding programs to breed for particular traits. Additionally, increased access to world trade meant that more breeders had access to the often superb Arabian and Berber horses, which had endurance, speed, and intelligence in fairly small packages. By selectively breeding, different groups were able to breed their horse type- like the Anglo Thoroughbred, the Dutch Warmblood, the Hanoverian, the Westphalian, the Trakehner, the Swedish Warmblood, etc. Essentially, military breeders would breed for the characteristics they believed would make for the best warhorse. Many of these qualities are similar- athleticism, a calm temper, solid bones, a solid color, a short back, a medium set neck, and an uphill build.
By keeping detailed records (stud books), horses could be developed for generations. Even today, it's possible to trace a Thoroughbred's background to one of the foundation horses of the breed.
The horses of antiquity were much less developed. Riding horses was still a fairly new idea (maybe less than 500-1000 years old). The earliest text we have that describes horsemanship comes from Xenophon, and the ideal horse he describes is drastically different from a modern European warmblood or warhorse.
Without bogging down on the details too much, his idea for a horse was something much shorter and leaner than what we would consider a horse today. The horse of antiquity would be called a pony today- and that's basically what they were. (Note- a pony is a small horse, NOT a baby horse).
They also did not really breed with as much detailed record keeping. So, horses were just horses- either a good horse, a bad horse, a horse I bought from this town, a horse he bought from that town.
To answer your question, yes, a modern horse was, in general, better. The modern horse was more specialized, more refined, and better suited for his job as a warhorse. He was drastically different physically from his counterpart in antiquity.
I can give you some sources if you'd like them. I can also link to some pictures showing the differences between early horses and modern horses if there is interest. Until then, I'm going to bed.