That is absolutely incredible. No clue what it was, but I have seen a Murray Grey cow with a similar marking. Hers were dapples that showed up when she was , and it almost looked like a sort of tortoise shell pattern - as in hexagon-like shapes.
I just did a little research and found someone who said that it is a rare genetic thing that usually appears later in life... But like, 3-4 years old later in life. That doesn't really apply to your or your friends horse.
I hope you can find out what it is. If it's a genetic thing that can be bred, I will be one of the first in line to buy a giraffe horse!
She is turned out 24/7, but can go in the stall if she wants. She is fed in whichever place she chooses out of 3... two stalls or the arena feeder. I let her pick - she earned it. Funny she changes where she wants to eat every day - just stands in front of whichever feeder strikes her fancy lol. Funny horsie. Anyway, it is starting to go away on her rump as her winter coat is starting to come in. When I got her, her winter coat was already in place. My girlfriend (her previous owner) hadn't seen it before on her though. So strange. I have a vet checking on it - but will likely say, "No clue" just like her vet and the other one mentioned here. Also wrote to some coat marking specialists back east to see what they think. Yeah if this marking could be bred, that would be amazing. But I'd want it on both sides :)
Oh, I would also document as much as possible and send the info off to different equine hospitals, equine vet schools and regular equine studies schools. They may be interested enough to come and do testing as well as evaluations. Posted via Mobile Device
Looking at the photo I am thinking NOT cool. In fact, what I see are different lengths of hair and the short hair between the spots looks shiny and the spot areas look long and like they need to shed.
Since another horse that used to stable with this horse also shows the pattern, I will guess that it is a fungus. It could be a form of rain rot fungus or something else. It may even be a fungus that find the horse a "dead end" host. If it returns I would talk to my vet about a skin scraping from the patches.
I'm inclined to think skin fungus as well, especially on your horse, where it almost looks like a chemical blistering (acid rain?? Reaction to the skin vs. fly product vs. standing with that side in the sun??) Wonder if the Vet doing a skin scraping and looking under microscope could help. WEIRD!!