A horse-drawn vehicle is a mechanized piece of equipment pulled by one horse or by a team of horses. These vehicles typically had two or four wheels and were used to carry passengers and/or a load. They were once common worldwide, but they have mostly been replaced by automobiles and other forms of self-propelled transport. General Horses were domesticated circa 3500 BCE. Prior to that oxen were used. Historically a wide variety of arrangements of horses and vehicles have been used, from chariot racing, which involved a small vehicle and four horses abreast, to horsecars or trollies, which used two horses to pull a car that was used in cities before electric trams were developed. A two-wheeled horse-drawn vehicle is a cart (see various types below, both for carrying people and for goods). Four-wheeled vehicles have many names – one for heavy loads is most commonly called a wagon. Very light carts and wagons can also be pulled by donkeys (much smaller than horses), ponies or mules. Other smaller animals are occasionally used, such as large dogs, llamas and goats (see draught animals). Heavy wagons, carts and agricultural implements can also be pulled by other large draught animals such as oxen, water buffalo, yaks or even camels and elephants. Vehicles pulled by one animal (or by animals in a single file) have two shafts which attach either side of the rearmost animal (the wheel animal or wheeler). Two animals in single file are referred to as a tandem arrangement, and three as a randem. Vehicles which are pulled by a pair (or by a team of several pairs) have a pole which attaches between the wheel pair. Other arrangements are also possible, for example, three or more abreast (a troika), a wheel pair with a single lead animal (a "unicorn"), or a wheel pair with three lead animals abreast (a "pickaxe"). Very heavy loads sometimes had an additional team behind to slow the vehicle down steep hills. Sometimes at a steep hill with frequent traffic, such a team would be hired to passing wagons to help them up or down the hill. Horse-drawn carriages have been in use for at least 3,500 years. Two-wheeled vehicles are balanced by the distribution of weight of the load (driver, passengers, and goods) over the axle, and then held level by the animal – this means that the shafts (or sometimes a pole for