I was recently told you can give a dog that ate smethng poisonous a mixture of half h2o2 and 1/2 water to make them throw up. They were speaking of the standard drugstore 3% grade.
But using h2o2 therapeutically is completely different. A rather small portion is mixed with water and I acts by oxidizing pathogens in the blood. I haven't read the links provided in this thread yet, but imagine they explain how it works.
I don't know this could just be mixed in water for pets, as I believe the extra oxygen is rather unstable and the drinking water would rather quickly become just water after the oxygen is released. It would kill any pathogens that were in the water itself though, I would think.