Horses' teeth are different, our teeth don't grow the dentine like theirs do. So I would think the tooth is nipped off to just above the dentine but above the core. Imagine how painful our teeth would be if the tops were filed? Owies!!!
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That's because the roots of horses' teeth go way deeper than ours, so you don't see the sensitive part of the tooth as it is so deep in their jaw. There's a whole cavity in the upper jaw that if full of the roots of the teeth when the horse is young, and they descend into the mouth as they wear, leaving the cavity behind.
The yellow bit in the centre of the teeth of older horses, the dental star, is where the nerve would have been while they were younger, but this gets filled with secondary dentine as they grow into the mouth. Hence where the term 'long in the tooth' comes from ;)
Also, all horses have canine teeth - just in many mares they never actually erupt (the 28% thing isn't strictly true, by the way, it's nearer 35%-40%. More than you would think). Wolf teeth are the real anomaly, and the ones that would be removed if they are in the way.
Teeth are so fun... and horse teeth, like the rest of the digestive system, are weirdly similar to rabbits rather than any other large herbivore. It's pretty cool.