(OOC: It's a Riding Dog, right, the strong kind of dog? They list a collie as a possibility, and female collies weigh about 50-65 pounds (female huskies weigh about the same too, but the third choice, St. Bernard, is much much heavier). If it is a female collie or husky, you should be fine carrying her, though she would use up pretty much half of your allowed weight limit herself. Remember that Phantom Steed lets the rider (no matter how much the rider weighs) fly for free, but above and beyond the rider's own body weight, you only get 100+ pounds or so)