When you take your horse's temperature, make sure you use a large animal rectal thermometer. These thermometers are extra thick, so they will be less likely to break if dropped. If you have one with a ring on the end, you can attach a small alligator clip with some nylon or fishing string, so the thermometer can be clipped to the horse's tail when you take it's temperature. This will keep the thermometer from breaking if the horse swishes its tail and knocks it out.
First, shake the thermometer down. Do this by holding the ring end between your finger and thumb. Shake it downward with short, snapping motions. Do this until the mercury is down to below 35 degrees. How low you shake it does not really matter unless the horse's temperature is way below normal. If this is the case, shake it all the way down. Lubricate it with petroleum jelly before inserting it.
To take the horse's temperature, stand on its left side, facing its rear, and set your hip so it is resting against the horse's leg. Grab the tail with your left hand, about six inches from the root. Hold the thermometer in you right hand. Slowly but firmly raise the tail up and to the side, and insert the thermometer. Push it in gently until only about half an inch is sticking out. Then, clip the alligator clamp on the horse's tail hair. Leave the thermometer in for about three minutes. When you remove it, be sure to unclamp the clip first. When you have removed it, wipe it through a few strands of the horse's tail hair to clean it before you read it.
The normal temperature for a horse is 37.0 to 38 degrees C. The Victorian Department of Primary Industries state that if your horse has temperature of 39 degrees C then you must stay at home and call a vet.