Any suggestions as to what this could be? (pic attached)

[_Libby_]

I found this small lump on my mares back leg just on the outside of the fetlock. Its appeared pretty much overnight and she hasnt had anything like it before?

Its hard and doesnt seem to be causing any pain.

Any ideas?

 

[AmyMay]

Could it be an abcess of some sort, caused by something like a thorn under the skin??