The story is the third movie, and an episode in the series. Interesting.
The "tower" honouring Ho-Oh was attacked and burned to the ground. All that wasn't destroyed was the bells. 3 Pokémon didn't make it out and were burned alive. Ho-Oh choose to save them and turned them into the beasts.
Ho-oh doesn't want to show himself to anyone and until humans and Pokémon reach that super-duper kumbiya balance. The dogs are supposed to watch us and see if that's happened.