What is the normal body temperature?

The normal body temperature of an adult horse is 99.5 to 100 degrees.
(Normal for a foal is 99 to 102 degrees)

The temperature is taken rectally. You can use either a bulb thermometer (the glass type with the little bulb of red mercury at the end) or a digital thermometer (my preference)

Lubricate the end of the thermometer. You can use Vasoline but a water based product such as KY jelly is best. With one hand, grip the horses tail firmly and raise the tail out of your way. With your other hand, gently insert the thermometer into the horses rectum about 2 to 3 inches.

For a bulb thermometer, wait 3 minutes before removing the thermometer. Digital thermometers normally beep when the accurate temperature has been taken.

After reading the temperature, don't forget to wash/sanitize the thermometer before storing it away for the next use.