It depends mainly on where you live but during the winter they should have a heavy blanket and during early spring and late fall they should have a sheet or light blanket.
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.
Horses typically need blankets during colder seasons, such as fall and winter, when temperatures drop below their comfort level. Blankets help keep them warm and maintain their body temperature. It's important to monitor the weather and your horse's condition to determine when a blanket is necessary.
Horses may need blankets during colder seasons such as fall and winter, particularly if they are clipped or have limited shelter. It's important to consider factors such as the horse's breed, coat thickness, and weather conditions when deciding whether to use a blanket. Always monitor your horse for signs of discomfort or overheating when using a blanket.
it depends on the weather,season, and the kind of blanket
Winter I should imagine.
winter
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.
Horses do not really need a blanket and it is actually unhealthy for them to wear one as you can read in the related link. The only time a horse really needs a blanket is if you clip them in the winter, robbing him of his protection from the cold. To tell if your horse is cold, touch his ears. If they are cold, he is cold. If not, leave him be without a blanket.
u should only put a blanket on during winter. later fall and early spring u can have a light blanket on. horses do grow there own winter coat that is (don't quote me on this but) 10x warmer than a blanket and you should let them grow that for a little before u put a blanket on them. also younger horses do not need a blanket they should grow there coats out.
Winter
winter and fall but mostly 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.