The tiny pinto stallion was born last Thursday, April 22, weighing only six pounds, and standing just 14 inches high. Einstein shows no signs of dwarfism, though.
Breeder Judy Smith, of Tiz A Miniature Horse Farm, in Barnstead, New Hampshire, said she thought the foal was dead when he was born.
Smith told Sky News: "I have been at this for 20 years plus, but I have never seen one this tiny or even close to it. At first he didn't move very much. We started rubbing him with a towel and he started to move around. He was so small!"
The little foal, named Einstein, shows no signs of dwarfism like the current record holder. Thumbelina currently holds the Guinness World Book of Records title for smallest horse at nine pounds.
Einstein's owner Charlie Cantrell said: "From the head to the legs - the length, width, height - everything is just beautiful about this little horse."
Co-owner Dr Rachel Wagner said: "Most of the ones that are really tiny are what we call dwarves and they have some sort of dysmorphic features, or features that aren't really normal or healthy. This little guy is like all horses - he's almost all leg."