Thank heaven that docking horses is illegal in the UK. It is seen an animal cruelty and anyone caught doing it unless for a medical reason, ends up in jail. There is no reason on earth for horses to be docked except to pander to man's vanity. Hopefully the rest of the world will come to its senses soon and horse docking will be relegated to the past.
I think that it is an old practice, that should go away! I so wish that my Percheron had his tail so that he could swat flies on his own, and because it would be beautiful, not this short clump of hair.
My 2 cents, and I know that it's not worth that! :)