A pony truck, in railway terminology, is a leading truck with only two wheels.
Its invention is generally credited to Levi Bissell, who devised one in 1857 and patented it the following year. Hence the term Bissel bogie (spelt with one 'l') or axle is used in continental Europe. In the UK, the term is Bissell truck with two 'l's.
Conservative locomotive builders in Bissell's native United States did not take to the design, and it was not implemented until the Eastern Counties Railway in the United Kingdom fitted one to their No. 248 in 1859. Pony trucks of similar design became very popular on British locomotives thereafter.
Pony truck Wikipedia
Partly because load was not equalized between the Bissell pony truck and the leading drivers, the pony truck did not become instantly popular in the US. Locomotives in the UK were generally not equalized, so it was not considered a problem there. John P. Laird was the first to attempt to equalize the pony truck with the drivers in 1857; he received a patent on the concept in 1866. Laird's design was complex and did not find favor, although he incorporated it on some locomotives he built or rebuilt, particularly on the Marietta and Cincinnati Railroad and later the Pennsylvania Railroad.
John H. Whetstone of Cincinnati, working for Niles and Company, was the next to devise a method for equalizing; in this scheme, the truck frame was itself the equalizing beam as well. Niles went bankrupt before the patent was granted, and no locomotive was ever fitted with this design.
A more successful scheme for equalizing the pony truck to the drivers was invented by William S. Hudson, superintendent of the Rogers Locomotive Works, and patented in 1864. In this design, a large equalizing lever linked the front truck with a transverse bar connected to the front spring hangers of the driving wheels. This design was an immediate success and was used on American-built locomotives until the end of steam building.
In the US, these trucks were known as lead trucks. A pony truck was the lead truck on a horse drawn rail car or trolley. A pony truck required a hitch to attach horses. The term applied to US Steam Locomotives after 1900 is considered archaic.