I have a chestnut gelding who - in the summer - has a dark brown dorsal stripe running along his back. (He also gets dark dapples, but that's aside from the point )
It's not very dominant, but you can still see that it's there.
Now, I know for sure he's not a red dun, for he's a dark chestnut horse. I don't know whether it's something about the sun or nutritional wise that brings it out in him, or maybe he carries some sort of gene, but I'm dying to know what's going on with him. O__O
I'd appreciate any help or ideas.