Historically, traveling menageries would often feature particularly small horses. All of which were confidently promoted as 'the smallest horse in the world'. And when traveling menageries fell from popularity early in the 20th century, this tradition of displaying the worlds (allegedly) smallest horse was continued with country fairs and some circuses.
There are many miniature lines of horses but the smallest specimens also tend to have some form of dwarfism. There are four kinds of equine dwarfism (achondroplasia, brachiocephalia, diastrophia, and hypochondrogenesis) and they all carry unacceptable consequences for the wealth and wellbeing of the affected animals. Miniature horses are sometimes not responsibly bred which leads to the proliferation of dwarfism in some lines.
All forms of dwarfism are recessive and require both parents to be carriers. But in the absence of tests there is no way to determine whether horses are carriers other than the foals they produce.
The horses are listed in approximate chronological order, from oldest to most recent examples.Even today, multiple horses lay claim to the title of "world's smallest horse".
Four years old? I don't think so; this horse does not look fully grown to me. Perhaps an example of side show exaggeration?
Teddy is listed as 27.5 inches high at the age of four years. I have to say he looks a bit younger than that to my eyes.
I am wondering if Teddy is also the unnamed 'world's smallest horse' in the picture below? It looks like the same horse to me and both photographs would date from around 1900.
And below that is Dick, advertised as the smallest entire horse in the world and owned by the Sir Robert Fossett circus.
Dackelpferd translates a "dachshund horse".
Tom Thumb and Admiral Dot (1907)
Dating from 1907 the postcard shown above depictsTom Thumb and Admiral Dot--also advertised as the smallest horses in the world. Their exact height or weight is not provided.
Tiny Tot (1910)
Tiny Tot was publicized as "the World's Smallest Horse". This picture dates from around 1910 and was distributed as a postcard. He was said to weigh 76 pounds and stand 28 inches high. He was owned by D. E. Clark of Cleveland of Ohio who is probably the man pictured here.
Black Beauty (2001)
At 18.5 inches tall, Black Beauty was recognized as the world's smallest adult horse from 2001-2007.
Thumbelina was recognized as the smallest living horse in 2007 by the Guinness Book of Records. At only 17 inches tall, Thumbelina (shown right) is clearly the tiniest of them all. This picture dates from 2008.
Koda is a miniature horse that is also a dwarf and is under 12 inches tall. This article discusses the serious health ossues associated with dwarfism, which is why most breeder will not deliberately select for it.
Einstein, shown below, is only 14 inches high. But he is not fully grown.