It is the Prophecy of God as given by God the Holy Spirit to Zecharia. The first portion has been fulfilled. The Second Coming and the Millenial reign has not yet occurred.
LOL If Christ riding a donkey into town fulfills the prophecy in your mind, then I am happy for you : ) The 'prophecy' didn't even mention Christ by name. I wonder if there was a Jewish king after Zecharia that didn't fulfill that Prophecy? Donkeys were the royal means of transportation.
"b. A donkey, a colt, the foal of a donkey: This seems to be a Hebrew expression of speech emphasizing that the animal is purebred - a truly magnificent, royal mount.
ii. “God had commanded the kings of Israel not to multiply horses. The kings who broke this command were miserable themselves, and scourgers to their people. Jesus came to fulfill the law. Had he in his title of king rode upon a horse, it would have been a breach of a positive command of God; therefore he rode upon an ass.” (Clarke)"
All the Kings of Israel rode asses : )