Dogs and cats have a very good instinct for essential oils and even seem to know what is good for them. As we all know, dogs especially, have a very strong sense of smell, more so than humans, so generally only use a minimum quantity of essential oil.
If you apply an oil to one hand that is a digestive aid oil, and a pesticide aid in the other, a dog with a sore tummy will know which hand has a digestive factor and lick it. Dogs have 200 million olfactory receptors, (20 times the number we have!) so the idea is when applying oils to a dog i.e for fleas let’s say, 1/10 is the rule - one drop to ten mls of vegetable carrier.
There are many oils which are useful to pets but my recommendation is to consult with an aromatherapist or check in with the vet before applying anything to the coat of an animal. Send me an email if you are ever unsure to firstname.lastname@example.org.
Oils to be aware of when burning/diffusing for yourself are: eucalyptus oil, tea tree oil, cinnamon, pennyroyal, peppermint, pine, sweet birch, wintergreen. These oils can be upsetting to pets (particularly kittens and birds).
If a blend has any of these oils in please be sure to have a well ventilated room, use much less oil (no more than 4 drops to a full water diffuser) and open a door for animals to remove themselves from the room if they are offended by the scent. They generally have a good intuition when they don’t like a scent. Ensure all of your oils and diffusers are out of reach for an animal (like a curious kitten) that it can’t knock it or lick it.
Remember, Google isn’t always right, it mostly scares you with the information out there, but it’s always safe to seek expert opinion if you are in doubt. I have many aromatherapy books which suggest useful oils for animals tried and tested, but just like humans, they have individual preferences and sensitivities.
A good reference for pet blends comes from a book called The Fragrant Pharmacy by Valerie Wornwood.
And again, just reach out if you’re unsure - better to be safe.