Is this possible for a horse's coat?

If a horse was born with a white spot on its back leg, chances are they will keep that spot for life. HOWEVER, some horses do go through a dramatic color change as they age. But I kinda doubt that a horse would gain spots due to neglect.

If a horse gets white spots that aren't permanent, and will go away after a few days, they are called birdcatcher spots. They usually show up in Thoroughbreds and Quarter Horses.

As for the rescue, She is probably sun-bleached. There are many ways to keep a black horse black. Anywhere from sunflower seeds to paprika, cinnamon to copper supplements. Some people bring their horses in during the day and turn them out at night (if possible), or they use a light weight sheet (like a fly sheet) during the summer.

Currently I have a black mare that is fading. At the moment, she is a milk chocolate color. Usually after I ride her, I rinse all the sweat off of her. Some think that sweat helps the fading process. This fall I am going to start giving her some extra zinc or copper, I haven't decided which yet.

I hope this helps!

Paints can change color their whole lives, it depends on the breeding. Lippizaners are born black and turn grey - white as they age.

My dad has two paints, one has stayed exactly the same from the day he was born and the other changes every year

There is also the cases of the few-spot Appy's that are sometimes born very dark and get fewer spots as they age (ie Mystic Warrior, friesian x appy, was almost black when he was born and now is mostly white with a few black spots)

As for fading color, that could be just bleaching out, which will go away in the winter. I have seen horses that are completely solid that as they got older got white dots all over their body. I'm not exactly sure what this is from, but I've seen it in a handful of horses.

Most horses fade as they get older. It's not because she's a rescue. The color of a rescue won't change just the quality of the coat. It sheds out and becomes brighter and shinier. Its normal to be changing color liek this.

I don't think that the black with white half dollar's will get spots.

Usually black horses fade from black to a reddish-brown when they are in the sun for long periods of time. The color will get lighter until they are almost a reddish bay color. It goes away if you keep them in stalls in the day and let out into pasture at night, although it does go away in the winter.

All of the theories that you mention are bunkum.

Some black horses will remain black, others get paler as they age and turn grey (white) between the age of 7 - 10.