To keep a freshly-washed tail looking immaculate overnight, I plait it loosely, secure the bottom with a rubber band, and then thread one leg of an old pair of tights onto the tail. I put the first rubber band just below the dock, and a few more further down the tail. Of course you mustn’t put anything constrictive on the dock area overnight, I have seen photos where this has happened and the poor horse has lost its entire dock.
This method -> keeps the tail perfectly clean (there’s usually poo all down the outside in the morning!) and most horses don’t mind it at all and don’t even attempt to swish it off.
(Btw, yes, this is an old pair of MY tights, and NO, i have no idea what I was thinking. Light grey tights with stripes across them?!?! Maybe they were a present…)