If dogs have a planet where they are the rulers, and humans have never been in control, than that planet can't possibly have anything like pugs or chihuahuas. These and other "toy" breeds can only exist because of inbreeding and genetic engineering directed by humans, in a manner that goes against the natural course of canine nature.
When Owen is trying to teach Hubble to play fetch and is throwing the ball for the other dogs, Nelly is always either at the back of the pack or not even seen. Italian Greyhounds, like Nelly, are built for speed and can run faster than most dogs. Most likely, if you were to throw a ball for a Bernese Mountain Dog (Shep, who was always in the front) and an Italian Greyhound, the greyhound would be way ahead of the Bernese Mountain Dog.
The goof items below may give away important plot points.
Near the end of the movie, when Owen stands on the hill while the flying saucer appears, there is a close-up on his smiling face. He is first looking over his right shoulder, there is a cutaway to his family, and when they return to Owen the mirror image is shown, with him looking over his left shoulder.