Found the below small bone in the Peace River,FL this week. After searching the forum and online I believe it is a medial phalanx. It measures 14mm x 22mm. The only similar size one I found a photo of was created and posted by @Harry Pristis. The one I felt was possible was that of Archaeohippus (3 toed horse). Last spring I found an incisor from a three toed horse in the same area.
I know @Shellseeker has done a good bit of research on these small horses and hope he can also offer an opinion.