Which is a disgusting mutilation, in my very humble opinion, along with ear-cropping, de-barking and the de-clawing of cats. (Stands back for tail-cropping enthusiasts to lecture on how the tails can get caught when the animals are hunting, swimming, fighting, etc.) And the tails are cut off living puppies, for God's sake, not dead horses!
Pee, it's no biggie on the world scale, and as I said, it's mostly a cultural thing - the Victorians were really into all kinds of memento mori, the hair from the dead being woven into belts, made up into mourning brooches and mourning rings, etc. Even pictures were made up from tiny snips of dead babies' locks. Personally, I've only got photos of my now-dead horses - I'm not sure I'd want to moon over one of their skulls, Yorick-style, no matter how well I knew it!