lol well as far as I know bryant makes it just on pet photography. BUT they do people and pet photography together. It's not all just pets. I dabble in doing people with their pets but not all that often.
I think most of us here agree that you need to do other types if you're going to get into the business, but I really don't think the big hitters are weddings. They CAN make you the most money in one session, but you can make just as much in others depending on what it is.
The Dogumentarian site that I had linked I'm pretty sure she has another job that she does regularly.
If pet photography was one of the things I wanted to get into really bad I could, but right now I'm happy with the little studio inside the petshop we got. One of the best things you can do like others mentioned is hit up the dog shows. Rent you out a booth or hand out leaflets/cards whatever you need to do to get your name out. Petshops and fundraisers are really great ways to use your knowledge too.