Trabag - Manchado Chestnut Arabian

The gene pool in Argentina may have been flooded with the introduction of other Arab lines........once a gene pool has been flooded, anything recessive can become virtually non existent. Therefore the odds of seeing it pop up are 1:1000,000..........if you could get a group of horses very closely related to this particular line and line breed them for years on end, this gene may show up again........make that gene pool shallow......

Exactly. Isolate a line that MAY have a new gene, and inbreed it until the family tree doesn't have branches. Not that I am advocating doing it, but that's how you do it. Turn the gene pool into a gene puddle

Oh gawd... They tried that in my little town called Newbrook.
Epic. Fail.
Or maybe horses are just more hardy then people.

I grew up in a small town of NZ natives and they were all 'related' in one way or another (let's not go there) but the cases of the women getting cancer and dying in their late 30's to early 40's was tremendously high......

Eesh.
These ones don't die (aside from the fact that for each year I went to the school here, one grade 12 student died - most drinking and driving) they seem to live forever. But they appear to be suffering from a severe case of Inbredinitis Stupiditis.
I can't wait until I'm rich and can move south.

^^^^^ Well that made me smile!!!
Maybe the horses actually arent purebred at all and someone once sneaked in another breed into this little gene pool and the colour pops up again from time to time?

IF the horses in Argentina are crossed...maybe. Trabag is the only documented Arabian to exhibit this colour - so if the TB Manchado (whom the colour was tagged for) was not actually purebred, we could maybe go with that.

I believe it is more likely that the color was among the founding stock brought to the country, and not slipped in later. Argentina was a big importer of colorful stock, back before stud books were common. There were British Hackney breeders who raised tobianos specifically for that market. Some made it into the early Hackney stud books, and one was exported to the US and went on to be a foundation horse for the Moroccan Spotted Horse, but many more were said to be sent to Argentina. It is not hard to believe that other colorful animals went there, too, and that remnants in the form of a hidden recessive remained after solid colors became more fashionable. In that way, the breeders may never have knowingly included this. All it would take is that they use local mares in the early stages of their purebred breeding programs, and it could from there become a part of each domestic breeding group there.

WS - somewhat off topic but since you queried environment as a cause and you're from Canada, do you remember talk of blue cattle found in one particular area of Quebec? This was quite a few years ago now. The cattle, as I recall, were Holsteins that apparently had a very noticeable blue in their coat colour. I don't know if they still exist today. The general concensus was that the colour resulted from their diet of whatever was growing in that region at that time.

I saw an article about the fungal infection that causes this . I was googling odd horse colors, or something like that and ran across the article.
It was either fungal or bacterial skin infection .