Hi Determined 1985:
A horse's temperature needs to be taken rectally, as it's hard to get them to hold a thermometer under their tongue! I recommend using a glass veterinary thermometer that has a small glass loop at the end. I prefer to attach a string to the glass loop, and then attach a clothespin at the end of the string. This serves as a great clip that can be attached to the base of the tail while you're waiting. You should leave the thermometer in the rectum for at least one and a half minutes. If you don't have access to a veterinary thermometer, a digital type will work as well, which can be purchased at your drug store. If the horse will safely allow it, place the tip of the thermometer into the rectum until you hear the digital beep. A small amount of lube or vaseline can help make the job a bit easier as well. Normal temperature for a horse is between 99 to 100.5.
Hope this helps!