This fake toe, dated between 1069 and 664 BCE, is likely the first practical prosthetic. The toe, which articulates, is still attached to a mummy currently on display at the Cairo Museum. From the BBC News:
A Manchester University team hope to prove that the leather and wood "Cairo toe" not only looked the part but also helped its owner walk.
They will test a replica in volunteers whose right big toe is missing.
If true, the toe will predate the currently considered earliest practical prosthesis – a fake leg from 300BC.