Using the Star K's kosher-for-Pesach pet food list, I finally purchased my pet food for Pesach. For eight days and three upset pet tummies, I spent $70 on Science Diet food. The other brand (Blue Wilderness) would have been $100. I'm a little shocked, as you might imagine. One more reason orthodoxy is expensive!
On the other hand, I'm incredibly amused that even though I am Ashkenazi, I can feed my pets kitniyot on Pesach, bwahaha... I guess I never thought about "possessing" kitniyot because I automatically group kitniyot with chametz.
If you're a fellow pet-owner, try to purchase somewhat more pet food than you'll need for Passover so that you can gradually introduce the new food to your pet before Pesach begins and then wean them off it gradually after Pesach is over. This will help reduce upset stomachs and the messes that can result from that. Your pets will thank you.
For a good general overview of pet food and halacha (I'll be writing a post on this in the nearish future), see Pesach and Halachic Issues with Pets.