Standing just 17 inches tall, she is never going to be a champion show-jumper.
In fact, the tiny mare is so small she would struggle to leap over a bucket.
But such things are of little concern for feisty Thumbelina who has just been officially recognised as the world's smallest horse.
The five-year-old received the title from the Guinness Book of Records after her astonished owners realised she was never going to grow any bigger.
She was born on a farm in America to a couple who specialise in breeding miniature horses.
These popular show horses usually weigh about 250lb and reach a height of 34 inches when they are fully grown.
But when Thumbelina was born, it was immediately clear she would never grow to this size.
At birth she weighed 8lb - the weight of many new-born babies - and eventually she grew to a mere 60lb.
Thumbelina's extraordinary size has been put down to dwarfism, which makes her a miniature of a miniature.
But despite this massive difference in size, it is feisty Thumbelina who rules the roost over the stallions and racehorses on her 150-acre farm.
'When she was born, she was so small we thought she wasn't going to make it,' said Michael Goessling, whose parents Kay and Paul bred the miniature horses.
'She weighed eight pounds when she came out and she looked very ill. We feared the worst.
'Because her legs are proportionally smaller than her body and her head, she has to wear orthopaedic fittings to straighten them a lot of the time.
'But we love her and wouldn't want her any other way.'
At a mere 17 inches tall (four hands), the mare measures up to the shins of the 'normal' horses in the paddock.
The Goessling family have bred miniature horses for the past 15 years on Goessling's Goose Creek Farm in St Louis, and these usually stand at 34 inches at the withers - the ridge between the two shoulder blades.
But the owners of the mini horse began to realise they may have bred a record-breaker when she stopped growing after a year.
'My parents have bred hundreds of miniature horses, but we have never had one as small as Thumbelina,' Mr Goessling said.
'She was just a complete fluke and we call her a mini mini.
'When she was young she found the dog kennels and decided she wanted to bed-in with the dogs, rather than with bigger horses.
'She spends all her time playing with the spaniels, but we have to try and stop her grazing on grass, because she is not allowed to eat too much.'
Thumbelina survives on a cup of grain and handful of hay, served twice-a-day.
Normal horses lives for about 35 years, but she is only likely to live up to the age of 17 because of her size.
She has the ability to become pregnant and give birth to foals, but her owners have decided not to allow this to happen.
Mr Goessling, 39, said: 'There could be complications during the pregnancy, so we think it is better to avoid the risks.
'And although we love Thumbelina, we do not think it is right that the gene which creates dwarfism in horses is carried on through future generations.'
The tiny mare has become sometime of a celebrity in her home town in America, but Mr Goessling insists they will never sell her, no matter what price is offered.
'She is too precious to us to sell,' he added. 'I think my parents would sell me before they part with Thumbelina.
'She has that special Wow factor, which you only get when you physically see how small she really is.'
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