The Torah portion of Vayishlach begins by relating that Yaakov sent angels to his brother Esav, informing him that he had "sojourned by Lavan and tarried till now." He then told him "I have acquired cattle, donkeys and sheep." The Midrash notes: "'Donkey' refers to King Moshiach, as the verse states [in reference to Moshiach]: 'A poor man, riding on a donkey.' "
With regard to Moshiach's arrival, the Gemara points out the following seeming contradiction: "One verse states that Moshiach will arrive 'on a cloud,' while another refers to 'a poor man, riding on a donkey.' " The Gemara explains: "If the Jewish people merit, he will arrive 'on a cloud,' if not, he will arrive as 'a poor man, riding on a donkey.'"
A donkey is used when the distance or terrain that has to be covered is too great or too steep for a person to make it on foot. The same is true in a spiritual sense.
In spiritual terms, "donkey" - in Hebrew chamor, related to the word chomer, materiality - refers to the corporeal. When a person refines and purifies his materiality, his soul attains a level that it could not reach on its own. This is in keeping with the inner meaning of the verse: "There is much increase by the strength of the ox," i.e., using the power of the animal soul in one's service of G-d vastly enhances the spiritual level of the G-dly soul.
Until the arrival of Moshiach, the most that can be accomplished is that the lower levels of the soul – those aspects that were clothed within the body – are elevated through one's service with the body, one's "donkey" or "ox."
However, with the arrival of Moshiach, when the superiority of the body over the soul shall be revealed, then even those soul levels that were too lofty to clothe themselves within a body will – along with the essence of the soul itself – be elevated as a result of the soul's spiritual service.
This is also alluded to in the saying of our Sages: "If the Jewish people merit [in Hebrew zachu], he will arrive 'on a cloud,' if not [lo zachu], he will arrive as 'a poor man, riding on a donkey.'" The Hebrew word zachu also refers to a state of elegance and refinement.
Thus, if "zachu," i.e., if man's spiritual service involves only spiritually refined matters, then the revelation in the Time to Come will be only "on a cloud" – emanating only from above.
But if the service also involves working with unrefined and corporeal matters – lo zachu – elevating and refining them as well, then the revelation will be "riding on a donkey." It will have additional spiritual impetus, for the "donkey" will lift the soul to a level that it never could attain on its own.
Yaakov thus said: "I have acquired an 'ox' and a 'donkey' - the heightened spiritual revelation of Moshiach in the manner that was due him for transforming the corporeal into the spiritual.
(Based on an address of the Lubavitcher Rebbe, Likkutei Sichos, Vol. I, pp. 69-73)