Essential oils, such as peppermint, are completely toxic to pets. These are toxic whether applied to the skin, diffused, or licked up in the event of a spill.
Diffusers that emit a lovely, nose-pleasing aroma may appear harmless, but they can be dangerous because they use water vapour to diffuse tiny oil droplets into the air. If used in a small space and/or for an extended period of time, inhaling diffused oils is known to cause negative respiratory effects in pets.
It is important to note that cats and dogs are far more sensitive to scents than humans. What you may consider a minor, fragrant scent may be overwhelming and harmful to an animal.