Temperature is a form of physical energy. It's the same for humans and animals, so any body thermometer for human use is suitable for dogs as well. Since the regulations and requirements for medical products for humans are often higher than those for animals, any thermometer intended for human use should be safe for a dog as well.
If you intended to use a thermometer that is not a body thermometer (like one to measure the air temperature outside) then I have to disappoint you. Since the range between healthy and unhealthy body temperature is so small, you need a thermometer with high accuracy and low measuring error. Most consumer grade air thermometers don't specify the accuracy and are not suited to take body temperature.
One thing to keep in mind is that the normal body temperature of dogs is higher than that for humans.
Some things to consider:
- When taking the body temperature of a dog rectally, most dogs react to the intrusion by sitting down. You must make sure your dog doesn't sit down on the thermometer by holding his tail high enough he cannot lower his bum to the ground.
- I wouldn't use the same thermometer on humans once you used it on a dog.
- There are less intrusive ways of measuring body temperature like infrared body thermometers. These can measure the temperature by pointing them into the ear or open mouth of the dog, but the measurement might not be as accurate as a rectal measurement.
You can read more about the advantages and disadvantages of both types of thermometers here. The article also includes instructions on how to measure a dog's temperature correctly.