They look like very thick chunks of dried skin. You could even mistake them for some sort of mineral (or perhaps a growth on a tree, heh).
Don't you think that looks awfully like a piece of flint? Of course, to touch it's much more like leather, but the appearance!
They grow on the back of horses legs, and when they're big enough they'll fall off, or be picked off in the course of grooming.
They're pretty stinky, but, for someone who is interested in aroma, they're also oddly compelling - kind of the way triple salted liquorice gets me: I think it's horrible, yet I can't help but want another piece.
I cut up some bits from four different horses, and put them into grain neutral spirit. I didn't have any plan for the resulting liquid, I just though it would be an interesting thing to do.
At this point I suppose a perfumer would go ahead and make up a blend or two, but I'm not a perfumer, I'm a drinks retailer, so instead I made some notes on the aromas. You'll be relieved to know I didn't absent-mindedly sip any of the liquids.
The base note across all four samples was waxed jackets (the UK brand I'm thinking of would be Barbour). All four also had an animalic, sweaty aroma, although, interestingly, in two of them this was unpleasant whereas in the other two it was really rather nice.
As I say, I'm not a perfumer, but I do think that a small fraction of horse chestnut could well add a leathery, animal note to a perfume.
If you're interested in flavour or aroma, I can most definitely recommend trying this for yourself. At the very least it's an excuse to go look at a horse.
My notes on four horse chestnut extracts
Martini (a nine year old)
Waxed jackets. It evolved a sour cheesy note, which reminded me of what you taste when you put the rind of a parmesan into a sauce, and once it's soft you eat it. Left in the glass the off notes vanished leaving a pleasant waxy-leathery aroma.
Yogi (a thirty-plus year old Shetland pony)
Stinky and cheesy, but also sweet. The most unpleasant of the four, and the least waxy-leathery.
Polar (a seven year old)
Hay-grassy & waxed jackets. Over time the grassy note went, to be replaced by the baseline waxed jacket note. The most pleasant of the four.
Unknown Horse #4
This one was the mildest of the samples. Alongside or over the baseline Barbour jacket there was a lifted, menthol, or mint note (or at least, something that tickles the nostrils). Over time an earthy note appeared.