Why should Tziyon rejoice so much?
Malbim: This is due to the new miracles that will be done constantly. He is "Tzadik v'Nosha" - saved via his virtue, and not via a big army.
Why does it specify Tziyon and Yerushalayim?
Radak: They are the capitol.
Malbim: Yerushalayim refers to the commoners.
What do we learn from "Hine Malkech Yavo Lach"?
Rashi: This can refer only to Melech ha'Mashi'ach - "u'Mashlo mi'Yam Ad Yam" the coming verse. We do not find such a ruler of Yisrael like this in Bayis Sheni!
Why does it say "Tzadik v'Nosha Hu"?
Rashi: He is saved via Hashem.
Radak: He is a Tzadik, and in his virtue he will be saved from the sword of Gog and Magog.
Is he poor?
Rashi, Targum Yonasan: He is humble. Radak - Oni is like Anav. And so it says "Lo Yitz'ak v'Lo Yisa...; Kaneh Ratzutz Lo Yishbor" (Yeshayah 42:2-3). Malbim - this is unlike a king saved via a big army, who is haughty.
Why does he ride on a donkey?
Rashi: This is amidst humility. Radak - it is not amidst lack [of a horse], for the entire world is in his Reshus! Rather, it is due to humility. Also, it is to show that Yisrael will not need horses or chariots - "v'Hichrati Rechev me'Efrayim v'Sus mi'Yrushalayim" (10).
Malbim: This is the opposite of riding on a horse, which is "Muchan l'Yom Milchamah" (Mishlei 21:31).
What is "Ayir"?
Rashi: This is like "va'Yarim Asarah" (Bereishis 32:16 - male donkeys). Radak - The matter is repeated in different words. It mention Ayir, for it is young, and best (Malbim - most honorable) to ride on, like it says about Bnei Ya'ir 1 "Rochevim al Sheloshim Ayarim" (Shoftim 10:4).
Our texts of Radak say Avtzan. Before the printing press, texts were copied by hand. Surely, a copier recalled that Avtzan had 30 sons, and mistakenly wrote Avtzan in place of Ya'ir. (PF)
Why does it say "Ben Asonos"?
Radak: [It has only one mother;] it is the son of one of the Asonos. The same applies to "va'Yikaver b'Arei Gil'ad" (Shoftim 12:7) - in one of the cities of Gil'ad 1 .
Malbim: it will not be an Ayir Ben Sus (pony), rather, a young donkey 2 .
This is unlike Yalkut Shimoni 68, which says that his limbs fell off and were buried in different cities. (PF)
Also Rashi (Rosh Hashanah 3) and Radak (Shoftim 10:4) say that also a pony is called Ayir.