your horse usually needs a blanket when its raining. Some people say when it is windy but, that is not true. Horses hairs stick up when the are cold to keep them warm (just like us).
it depends on the weather,season, and the kind of blanket
Winter I should imagine.
winter
A horse should have a blanket in fall or winter to keep it warm.
There are two answers to your questions:If you are asking "if you need a blanket under a saddle" then, yes. It is to protect a horses back.But, if you are asking "if you need a full body blanket to keep a horse warm" then, no. Unless you are in Norway and you have a very thin coated horse.
If you blanket a horse at the beginning of Autumn then it has to stay on until warmer weather comes around again as the blanket prevents the horse from growing it's winter coat. Without the the natural hair coat or the blanket the horse could become susceptible to influenza. You can also blanket a horse with a thin blanket in the summer. This keeps the flies away and prevents the horses coat bleaching in the sun.
winter and fall but mostly winter
Winter
Usually you'd get your horse a new blanket in late fall or early winter. Or what ever the coldest season for your region is.
The answer is pretty simple... WINTER!Winter
you can't. the blanket will only go on your horse when the horse is sick.
water blanket and food so he/or she can stay warm and be not starving