The Hansom cab, as preferred by Sherlock Holmes as a technique for getting around the city, is, or rather was, a pony drawn carriage. Structured and protected by one Joseph Hansom, a modeler from York, in 1834.
Hansom understood that there was a requirement for a lighter, increasingly maneuverable, kind of transport than the Hackney carriage at that point.
The Hackney carriage was substantially more awkward, being a four wheeled carriage, drawn by a couple of ponies, and seating six individuals. On the other hand, the Hansom was a lot lighter, just two wheels, and just required one pony to pull it’s heap of two travelers and driver.