Wow, this case generated a lot of interest, with requests for the answer when I didn't post yesterday. My apologies for leaving you all hanging!
This is a case of a non-parasitic fiber that had been transcutaneously implanted and migrated under the skin due to physical pressure (i.e. walking). Although the presentation was suggestive of a cutaneous parasitic infection such as cutaneous larva migrans (CLM), the length of the object and dark color were not consistent with any human or zoonotic parasite. Several readers pointed out specifically that the larvae that cause CLM are microscopic and therefore would not be visible to the naked eye. Also, the path formed by a migrating worm would be more serpiginous rather than the semi-lunar pattern observed here. Other subcutaneous or intra-epidermal/dermal worms such as Loa loa, zoonotic microfilariae, and Dracunculus medinensis were also suggested but rightly discarded by readers because of the size and color of this object. There is no parasite, to my knowledge, that would have this appearance and be present in this location in human skin.
One intriguing suggestion was that this was cutaneous pili migrans (CPM) - an excellent thought. CPM is a phenomenon in which a hair grows within the skin rather than up and out of the skin, and is seen as long dark object just below the skin's surface. This is most common on hair-bearing regions of the body and would be unusual on the sole of the foot. However, removal of the object would be required to rule out CPM. Therefore, this is exactly what we did. Here are photographs of the object that we removed (cut in half):
Thanks again to Emily Hall for donating this fascinating case!