What is this "lump" on my horses fetlock joints?

[goussberry]

It looks and feels like a chestnut, but it's smaller and sticks out more, and it's on the fetlock joint on all 4 legs. Any idea what it is?

 

[Meowy Catkin]

Ergot?

 

[spacefaer]

Ergot

In prehistoric times, horses were the size of a dog, and had 5 toes. As the horse grew and developed, the toes withdrew up the leg - the 2 splint bones were toes, the ergot and chestnut were another 2, and effectively the modern horse is standing on the tip of one toe! Evolution is amazing!

 

[Meowy Catkin]

I've not seen any conclusive evidence that chestnuts and ergots are vestigial toes.

Incidentally, wild asses and donkeys are more like zebras than domestic horses in lacking ‘chestnuts’ (those weird, horny excrescences on the inside surfaces of the limbs) on the hindlimbs, an observation consistent with cladograms which show the caballine or caballid or caballoid lineage (domestic horses, European forest horses and so on) to be outside an ass + zebra clade. Nobody seems to know what the ‘chestnuts’ are: I’ve heard horsey people say that they’re vestigial remains of digits – Dent (1972) says this – but this can’t be true since they’re not associated with the metacarpus or metatarsus, the only places where digits occur.

http://blogs.scientificamerican.com/tetrapod-zoology/fantastic-asses/