if you just pause on frame one, that weird thing on the right hand side is the back half of the hero dog, but it got the leg orientation wrong, so on the next frame it breaks them into two to make a plausible frame where it's now two dogs. These video generators basically take a frame and then the prompt and try and just predict the next frame, so if it bungles the first frame in any way like this, it'll try and resolve it. The best solve was that it was two dogs with one slightly off camera. This is a good tell for AI as well, is edge details will be indecisive like this because there's so many ways that something entering the frame could resolve (it could be an ostrich leaving the frame, or a feather fan waving on the edge, or a second dog backing into the frame), so it ends up being an indecisive fluff.

That sir is the "right" side.

Right