Beagles and other hounds aren't the only rabbit running breeds, they're just the most popular. A guy I know who would beg to differ runs them with beagles when he hunts with his buds, but his hands down favorite for when he hunts bunnies alone is his Jack Russell terrier. He isn't the first person I've known that used a breed other than hounds to run rabbits, many breeds of terrier will run them effectively. Obviously a more sedate breed such as english bulldogs and other short legged dogs won't work, but hounds are not the only choice.