There is no right or wrong answer....though some people may give their opinion AS IF it was written by the finger of God!
It is what it is: owner preference. There is no set temperature whereby a horse MUST be blanketed, nor is there a rule whereby NO HORSE should ever be blanketed.
If you are asking for preferences, we prefer to blanket our horses. Shoot us, kill us, tie us up and torture us.. but, that is what we do.
Our rules are:
Horses come in during the night during the winter due to coyotes and bobcats...and the decrease in their natural food sources during this time of year. Wouldn't want them to attack one of our horses due to their starvation overcoming their natural instinct to go for smaller prey.
During the day, we blanket both our OTTB and our DRAFT with a sheet when the temps dip below 40 degrees. They live on the top of a mountain and it is almost always breezy up there. When it is raining, they are sheeted at 60 degrees or below. They are blanketed for snow.....we use a medium weight blanket for that. We do have a heavy weight blanket for the OTTB for when the temps dip into the teens or below zero. The draft does not have a heavy weight blanket as her medium weight blanket is sufficient for her due to her extreme fatness and hairyness