You should put a rug on when it starts to get cold. I would use a heavyweight rug as it keeps your horse warm and cozy throughout the winter.

I hope this helped!

Hi thanks for all the help!!! it did help :) Thank you all!!! x

like already been said, it depends on the type of horse and whether or not they are clipped.  snow is no problem, its getting wet thats the issue and can cause problems.  in cold (less than 5 C), wet weather, always make sure that your horse either has shelter and therefore the opportunity to get dry, or a rug.  my yearling (1/2tb 1/4shire 1/4cob) spent his first winter with no rug living out 24/7 and he was fine.

also, hay keeps their metabolism high and keeps them warm from their gut.  i never put on a rug at the beginning of winter to let them grow their coats (aside from the tb who has a really feable coat anyway!).  if a horse does get wet, give them a quick groom with a curry comb to fluff up their coat when they are dry again, and help them preserve more heat.

most of it is your own judgement, some people feel better to rug up their horses as much as possible, but sweating under a rug during the day can make them colder at night then they would be without it