Here at Pets.ie, we have both cats and dogs as do many of our customers. Spending a little time with either, will allow an existing pet to familiarise themselves with chickens.
Cats
Cats are normally no issue, as chickens are usually bigger than them. However, it is still a very good idea to keep an eye for the first few days when your new chickens are settling in to your garden. We have six cats here at Pets who have never shown any interest in fully grown chickens, come to think that behaviour is exactly the same sort of interest they show in me!
Cats like to do their own thing, and when you have one with hens it will most likely be the same. Our cats sometimes love to go in to the hen house and sleep in the nesting boxes, much to the annoyance of our hens I might add.
Dogs
Dogs can take a little bit more time, but dogs are easier to train and by letting them see the chickens while they are on a lead and telling the dog what is right and wrong usually sorts it quickly. Soon your dog and your chickens will be best friends and living in harmony. We have a German Shepherd, a Collie and a Cockapoo terrier who all get along with our flock really well. I have seen the hens go right up to the German Shepherd's food and eat from it.
Unless dogs are familiar with the chickens and the coop area, it is very important to supervise. Dogs can harm both the chickens and do a lot of damage to the coop and run area if they have not been accustomed to the chickens.