Going to talk about a specific breed - because what's going on with them is really cool.
This is an Akhal-Teke stallion named Ikon (image source Ulruppelt via Wikimeida Commons) and as you can see he is really shiny.
But how does he look like he was spun out of metal like that? Palominos are normally shiny, but this guy takes shiny one step further. The reason is that this breed has weird hair.
The opaque core of the hair is narrower or even absent, allowing the light to shine through the hair - except it refracts it a little to give that shimmery golden look. Black Akhal-Tekes can look blue or even purple.
It is very rarely seen in other breeds, and appears to be caused by a recessive gene. It occasionally shows up in Thoroughbreds, Warmbloods, Arabians and Quarter Horses - and probably indicates some distant steppe ancestry. (And of course in the occasional horse of mixed/uncertain breeding).