Yeah! I would just stick some vinegar into the spray bottle and that does the job, but if you want add some tea tree oil until you can get a faint smell off it and it will smell better than the vinegar alone!
Feed the Cider Vinegar to the horse
or use it on mudfever
As a rinse to help make horse shiny
Tea tree oil is great for lots of things, swish some around in the rinse water when you wash your horses tail
Dilute and use with a cloth to wipe out nostrils, under tail - a couple of drops of detergent helps to imulsify it with the water.
I tend to mix the Tea Tree in with a carrier oil (something harmless like Olive oil) and, using a sponge, wipe across the coat - keeps flies & midges away, and the oil makes the coat nice and shiney plus puts off the midges from biting! I do my boy's legs particularly, he's a traddie cob and gets a bit feather-mitey.
You can also mix a few drops of Tea Tree oil with a mild shampoo like baby shampoo, and use on horses and dogs. Supposed to repel fleas and be anti-microbial and anti-bacterial.
Cider vinegar is supposed to be really good for horses, gives a nice shine to the coat and keeps them in condition. Also I add a half-teaspoonful to the chooks drinking water as its supposed to control internal vermin plus helps keep them healthy!
My two have a splash of cider vinegar in their feed occasionally.
There's probably a whole lot of other uses, but its just I can't think of 'em right now!