When I was 8 my 15-year-old sister and her friend made a bet on my ability to read and pronounce the word "eohippus". She might have lost her $2 if I had been less horse crazy as child, but she surrendered her winnings to me after I read it, pronounced it and defined it.
Jumping ahead some 44,990,000 years I want to talk about the four forefathers of the modern horse. Tonka's admirers often comment that it seems maladapted for a horse not to be able to eat grass, but it makes sense when you consider their domestic history.
Pony I: Developed in the cool, wet climes of northwestern Europe where vegetation is short, hardy and not accustomed to sunlight. Accordingly, these horses were small, hardy and shaggy. Their closest living relative is the modern Shetland pony.
Pony II: Developed on the cool, arid steppe between Hungary and Mongolia, which is dominated by short, dry grasses and small shrubs. These horses were larger than Pony I and more resistant to extreme cold. Their closest living relative (though endangered) is Przewalski's Horse (and they were the source of today's dun colouration).
Horse III: Developed in the warm, semideserts of central Asia and eastern Europe where the vegetation is sparse and dry. These drought-proof horses were larger and longer than Pony II, and much less hairy. Their closest living relative is the modern Andalusian horse.
Horse IV: Developed in the hot deserts of western Asia where any vegetation is sparse. They were smaller than Horse III -- fine-boned, thin-skinned and fleet-footed. Their closest living relative is the Caspian Pony, rediscovered in northern Iran during the 1960s. Modern Arabians are also descendant from Horse IV.
People may associate horses with the lush grassy plains of North America (the ancestral home of cows), but their diet should be much less rich if evolutionary factors are considered. I am really glad that most modern horses are able to digest what we are able to feed them, but I am not surprised that so many suffer from nutritional regimes so far removed from their historical diets.