Einstein is just three days old, after being born on Friday at a farm in Barnstead, New Hampshire.
It's thought that Einstein could lay a claim to the title of the world's lightest foal - his 6lb weight at birth being normal for a human baby, but not a horse, even a miniature breed like Einstein.
Unlike other miniature horses - notably the current claimant of the world's smallest horse title, Thumbelina - Einstein doesn't show any signs of dawrfism.
Judy Smith, the owner of the Tiz A Miniature Horse Farm where he was born, at first feared that he was dead when his mother, Tiz Fenisse, gave birth.
'I have been at this for 20 years plus but I have never seen one this tiny or even close to it,' she said.
Dr Rachel Wagner, Einstein's co-owner, 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.'
