Keep your horse warm
Hypothermia will reduce their interest in food or water and their body will put a lot of energy into just keeping warm.
Though please do so responsibly as over-rugging horses is another issue all in itself! Our native breeds and unclipped hairy horses are much more robust than most people realise.
*As a rule of thumb, feel through their hair with an un-gloved hand down to skin level and if warm to the touch, they are fine.