I'm probably being daft, but...

[Casey76]

My new mare has a weird swelling just in front of her flank/hip whirl on each side.

My old horse didnt have one and my youngster doesn't have one.

Is it food related do you think? (She is on 100%hay only)

I took a couple if photos, but it is difficult to see

(Just after the shine line)

(Just after the swell of her belly)

(Just before the shine line from the other side)

 

[Cinnamontoast]

Happy to be corrected, but my boy has this and I think it's just his conformation. Sometimes I can't see it but if he's huffing and puffing, I see it. White line on dark flank: