Avrohom went to the Akeida on a Chamor [donkey], Yisachar is compared to a Chamor, Moshe took his family on Chamor to Mitzrayim, Bilam got blasted by his Chamor, Pinchos ben Yair's Chamor wouldn't eat Tevel, and Moshiach will arrive on a Chamor. What is it with the Chamor?
There is no harder working, obedient, and modest animal like a Donkey. It carries its load without fuss with its great strength. Even when it lies down it doesn't throw off its load. It never uses this strength to fight or rebel against the wishes of a human. This is its nature. It never worries about its food because it always knows it will be rewarded by its owner who needs to keep his trusty friend nice and strong.
The K'naf Renanim says that the Chamor is a reminder to us to put aside our own desires and subjugate ours wishes to that of our Master. "Bitul" was Avrohom's reaction to Hashem's command to slaughter his son. Yisachar is completely devoted to Torah. Bilam had to be told off by a Donkey who was practicing Bitul, because he didn't know how to be "Mivatel" himself. Moshiach will arrive when Klal Yisroel understands that we need to be Mivatel ourselves to Hashem and he will take care of us.
The Donkey sings: "Licha Hashem HaGedula, V'Hagevura, V'Hatiferes..." To you Hashem are all the virtues and powers. We look to you and are Mivatel ourselves.