There are a few reputable brands, such as DoTerra and Young Living. I personally have chosen to use DoTerra for many reasons. Most brands on the market, even if they say they are pure and therapeutic, are not. In short, plants need to be grown where they thrive best, they need to be harvested at certain lifecycles and at certain times of day and in specific ways. Once the farmer harvests the plants, they need to be distilled carefully and stored carefully. Unfortunately, many farmers are approached by unethical buyers who then take the harvest, pay a minimal amount to the farmer, and distill the plants in comparable ways and add synthetic fragrances to it to create more yield and therefore more money for them. They sell the distilled product to companies who bottle them and distribute them, and often label them as pure and therapeutic, neither of which is correct. 

Essential oil brands need to guarantee the efficacy and safety of their product by forming relationships with their growers across the world where the plants thrive best, they need to know how the plants are grown and harvested, and they need to be in control of the distillation process and storage after distillation to assure best practices and that there is no adulteration of other harmful chemicals being added to the essential oils, and even then the batches should be tested for purity and contaminants.  Those test results need to be made public so that the buyers know exactly what is in their bottle of essential oil. It's a complicated and complex process, and it's a lot to ask many essential oil companies to do, and that's why there are only a few brands on the market we can trust for use with our pets.