i have a 16yr old norwegian fjord horse who recently started to lather up after a very short ride at a walk on a cool day october day
You say a short ride but a short ride can be a very energetic ride to include jumping and cantering etc ... It may just be his age but check his diet, is he getting the right stuff. Try to get him to drink more water and put salts in it or salt in his feed to replace the ones he's sweated out. Being a Fjord I should think that his coat is rather thick and made do well to have his tummy and neck clipped. With the information you have set up, there is not much else I can add.
He could be out of shape, or it could be used to the cold weather and when he gets his muscles going, it could be very hot, or his winter coat, or a combination of the three! I wouldn't worry about it yet, unless he starts to act differently than usual, other than this. But if you feel uncomfortable, call the vet out!
That isn't a lot of information to go on, but my first thought is have a vet check your horse for Cushing's.
