World's Smallest Horse - Thumbelina is a miniature horse, which is also known as a mini. She is extra tiny because she is also a dwarf and that makes her a mini. That is why Thumbelina is the world’s smallest horse and one of the things that makes her very special. Thumbelina is a Chestnut Mare, which means she is a girl horse with a dark brown coat. Her coat is very soft and covers her whole body. In the winter, Thumbelina coat gets extra long and fluffy.