I'm starting this becuase the housepets page on TvTropes didn't have a WMG page. Basically, you can make the most bizarre assumptions about the comic. It's like theorizing about the comic without any sanity. Feel free to add or contrast to other people's entries.

I'll start off: Max and Peanut are brothers and Fido and Sabrina are their parents. That would explain Peanuts attraction towards cats. Plus, both Max and Sabrina have black fur.

You see?

Now you try.

1.) TV Tropes is a wiki, so if it doesn't have something you can always add it. For instance, I was sad that Wizard People, Dear Reader didn't have a page, so I made one myself. You can do the same thing here. In fact, it'd actually be awesome if you did.

2.) Part of the fun of WMG is that the guesses are usually possible. I really doubt a dog and a cat will be procreating.