9:9 behold, thy King. This is clearly a Messianic prophecy, quoted as such in Matthew 21:4-5; and John 12:14-15. The event is also described in Mark and Luke, all four writers clearly regarding it as of profound significance.
9:9 having salvation. The King who comes is fully righteous and able to bring salvation. Nevertheless, He is also “lowly” (or “afflicted”) and will enter Jerusalem in a manner not like kings would, with chariots and horses, but on a lowly donkey, thus identifying Himself with the poor.
9:9 foal of an ass. This terminology indicates the ass on which He would ride is not yet broken, for it is still following its mother. Yet, when the time came, the unbroken ass would willingly obey its Creator.