In horses, the most distal bone in their hindlimb (i. e. the one that they stand on), is homologous to what bone in humans?

The correct answer is metatarsal bones or metatarsus.

A homologous structure refers to an illustration of a bone or an organ, which appears in distinct animals, demonstrating anatomical similarities suggesting descency from a common ancestor. Or it can be said, that when very distinct animals exhibit bones, which seems very identical in function or form and seem to be associated.

The metatarsus or metatarsal bones refer to a combination of five lengthened bones in the foot, situated between the tarsal bones of the mid and hind foot and the phalanges of the toes.