Wiki says yes: https://en.m.wikipedia.org/wiki/Horse_hoof

“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.”

Oh, but it’s not a single finger! It’s more like if you cupped your fingers together so they were all touching and then the skin on all of them grew together and then the “nail” on your finger cluster grew out thick enough for you to stand on it!

Yes it is the middle finger/toe.

http://rspb.royalsocietypublishing.org/content/284/1861/20171174

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.

My vet tech partner says: yeah it’s the middle finger. They have splint bones on either side which are remnants from the other fingers.