I was responding to enemytosleep's answer, and I realized I had a lot to say, so I just decided to post my own answer.
I don't like this. In a legal sense, it would seem to me that labeling a pet owner as "guardian" would implicate the responsibilities of a guardian of a human child, and also the rights that child has. And we all know that animals' rights aren't nearly as broad or strong as those children enjoy.
Bottom line: animals are property. They are a very special subset of property, but they are property nonetheless. You can legally buy and sell them, which pretty much automatically makes something property right there. Animals enjoy rights that inanimate objects do not possess, but in a legal sense an animal is much more similar to a piece of furniture than a human child.
That's the way it is, and it's a good system. Animals are owned, but they have various statutes to protect them because we recognize that they are living, breathing creatures. Could the penalties for animal cruelty be steeper? Yes. Could the enforcement be better? Probably. But inserting the word "guardian" into the master/pet relationship won't do a damn thing except misconstrue the essential nature of that relationship. If you ask me, animal rights groups should focus on fixing actual problems rather than playing around with words.