According to veterinarians, a horse's temperature can vary somewhat with the season. During the winter, it is not uncommon for the temperature to drop to as low as 97. Usually, this is not a cause for concern that the temperature is that low, but rather, it makes it difficult to determine if the horse is running a fever from an infection.
Cold weather and horse vital signs
During the winter, a horse's temperature may drop to as low as 97 making it difficult to determine if the horse is running a fever from an infection since the cold can mask a temperature that would indicate an illness.
During the winter, any temperature above about 100.5 should be suspect, with average fevers normally running from 101.5 up to 104. The summer heat, as well as any exercise, can often raise the core temperature upward even without a fever. This must be taken into account when the assessment is made.
A race or show horse, after intense competition, can have a core temperature up to 105. Even at rest, in the summer heat under a tree, a temperature of 101 would not be considered abnormal. So events preceding the temperature must be taken into account before it is interpreted.
A high fever doesn't always indicate a severe condition, but if the temperature is over 102 F, you should call your veterinarian.
For more information: Your Horse's Vital Signs