"It is a complex structure surrounding the distal phalanx of the 3rd digit (digit III of the basic pentadactyl limb of vertebrates, evolved into a single weight-bearing digit in horses) of each of the four limbs, which is covered by soft tissue and keratinised (cornified) matter."
Horses have 5 fingers/toes in their genetic, but only the middle ones develop. The reason is that one finger/toe has advantages for a huge weight and wanting to run quite fast.
If this paper is anything to go by: yes, the hoof is the "middle" finger, with difficult to recognise vestiges of digits to the left and right still present.