I just saw your post about the heave lines, and my gelding that I mentioned in my last post has the same lines; so I suspect that is what they are.
For my mare, they didn't seem to be there when I first got her (pic below), but she was about 200 lbs overweight according to the vet when I got her (she'd been rehabbed at a foster home who put the weight on her because she was so skinny when she was rescued by the SPCA). So may have been hidden by her rotundness
...but the trainer I had at that time I believe moved her along a little more quickly than I would have liked (but I was a brand new horse owner at the time and had no idea what I was doing, so I trusted the trainer). So maybe the lines developed because of the strenuous activity she was asked to do while so out of shape?
Here's one of her in 2009 though...and I can't really see it except in the action shot:
And then 2010...(after she was off the molasses and soy)
So now I can't tell if that's what it is, or whether it's just muscle definition in general with her being fit?
She never coughs and hasn't since I got her, and doesn't "roar" when being worked or anything like that either...