The wearing of masks is a new normal. If we humans are still getting used to the appearance of the face mask in public, what do our pets thinks? Anything visually new or distracting can incite fear in our dog, cat, horse, pet bird, or other animal we socially interact with frequently.
Dogs may exhibit postures of fear, vocalize, move away, cower, or even growl. Cats may startle, run, vocalize, or hide. Our beloved pets' level of confidence and security is in part linked to daily ritual. Changes in environment, caregiver, and appearance can be scary. We can help your pet adjust to that change and diminish the nervous behavior that a mask may cause.
For dogs, we want to start by showing your dog a mask and offering a treat reward. You can hold it up to your face then back to your lap while offering that favorite treat. If your pet exhibits nervous behavior, then slow the process down or even stop the training session. Eventually you want to be able to hold the mask to your face and remove it repeatedly. Follow this step until you are wearing a mask while petting your dog, with the goal of moving around your home and outdoors all while your pet is cool, calm, and collected. Talking to them during this process can help reassure them that they still know you, even with the mask on.
Once your dog is used to this you can hit the streets. On walks have treats ready and your basic sit-stay training on point. If your pet is exhibiting nervous behavior when you encounter a mask wearer, put him or her in a sit, distract with a preferred treat, and allow that person to pass you. If possible, cross the street yourself before the sit and wait. Then with an energetic "good dog" proceed on your walk. This is the basic training tenant of all things that are big and scary for dogs.
For cats that may be more aggressively or fearfully reactive the training is similar. With cats, it may be more useful to start by wearing the mask. Periodically leave a high value treat near your cat while continuing your normal home routine. Let your cat investigate you. Do not force social interaction on your pet. Again, talking during this time will help the cat remember that it is still you under the new mask.
Horses and the similar hoofed animals may exhibit the same spectrum of behaviors as dogs and cats. You will take the same slow approach to desensitization. If your horse is not permitted treats, use the same praise that you do while riding or training. When possible spend more time working in your barn or paddock while wearing a mask and ignoring your horse. Play "hard to get" so your hayburner has to seek you out to investigate your change in appearance.
These are the very basic principles of desensitizing your pet to a visual stimulus.