Chestnuts? What are they & why does my horse have them?

Chestnuts are remnants of the pre-evolutionary leg and foot structure of Eohippus, considered vestigial due to their loss of purpose in the evolution process.  

‘Vestigial’ refers to something that has lost its purpose, as part of the evolution process.

Chestnuts are firm, fleshy growths located above the knees on the front legs and below the inside of the hock on the hind legs of horses. Similar to human fingerprints, chestnuts are unique in shape and composition for each horse, with growth rates varying among individuals. 

Some horse breeds lack chestnuts entirely, while donkeys and zebras only have chestnuts on their front legs.