Mammals
Both dogs and bunnies are mammals. Mammals are a class of animals that are distinguished by the presence of fur and the production of milk (in females). All mammals, including dogs and rabbits, are warm blooded and vertebrates (meaning they have back bones). Certain internal structures (like a single lower jaw bone and the presence of a diaphragm) are also common traits among mammals.
Size
Although dogs and rabbits come in a wide variety of sizes, most dogs tend to be significantly larger than most bunnies. The European common rabbit, one of the most common bunnies in the world, only grows up to 16 inches long and tends to weigh only 2 to 3 lbs. Even the Chihuahua, smallest of the small dogs, grows significantly larger (generally weighing between 6 and 8 lbs).
Pets
Both dogs and bunnies have been domesticated and are commonly kept as pets. Because of the wide variation in both species, the specific breed of dog or bunny right for you depends on your living situation among other things. Bunnies can be bought at most pet stores, but dogs often have to be adopted at a shelter.
Diet
Dogs are carnivores, meaning they eat primarily meat while bunnies are herbivores, meaning they eat primarily plants. Most dog food is primarily made of various meats (e.g., beef, chicken and turkey), but domesticated bunnies are generally fed lettuce leaves or some other plant. Bunnies in the wild are often thought of as pests because they tend to eat the garden plants.
Reproduction
One of the major defining features of bunnies is the speed at which they reproduce. Their gestation period is only 30 days long, meaning that they can have several litters within their breeding period (February to October). Rabbits tend to have five to eight young per litter. Dogs have a gestation period of roughly nine weeks. They also only breed at two times during the year (when they are in heat). Litter size can vary greatly, with some breeds only producing one or two puppies per litter while other breeds can produce up to 10 in one litter.