If you think it's hard to cast a human being for a part in a movie, try casting a dog. You can at least give feedback to a human and ask them to adjust their performance a little to suit what you're looking for. There's no way of telling a dog what they're getting wrong, and they'll just do what they want to do when the camera starts rolling.
Over 100 dogs were seen and considered for the role of Toto, who might just be the second most famous fictional dog of all time after Lassie, but the producers eventually settled on a Cairn Terrier called Terry, who we hope you all agree was adorable. We don't know how Terry felt about being a film star, but his owner was so thrilled that his name was permanently changed to Toto after he got the part.