The top three hierarchy of why people hire a professional photographer in the portrait/social sector goes brides, babies and pets, in that order. This data came out of a Kodak survey at least 15 years ago and I don't believe it's changed at all. What has changed is the growing number of pets in the world.
Just for the U.S. market: Data published on the ASPCA site over the last year or so shows "70-80 million dogs and 74-96 million cats are owned in the United States. Approximately 37-47% of all households in the United States have a dog, and 30-37% have a cat. (Source: APPA)."
It's a simple point - if pets aren't in your repertoire, maybe it's time to consider. At the very least, expect them to be in the wedding party now and then!