Why is Nebuchadnetzar called "Avdi"?
Rashi: He will do My desire, to pay (punish) those who hate Me.
How did Hashem give Chayos to Nebuchadnetzar to serve him?
Rashi (28:14) and Radak, from Shabbos 150a: He rode on a lion, and tied a serpent on his head.
Radak #1: This is like "v'Chayas ha'Sadeh Hashlemah Lach" (Iyov 5:23).
Radak #2: At the time that he lived among animals, they brought to him food. This is like a Drash. At the time he ate grass 1 , like it says "v'Isva k'Sorin Lach Yeta'amun" (Daniel 4:22)!
Malbim: He used vicious animals in war, like elephants and lions, which were used in wars in ancient days.
So there was no need for animals to bring to him food. (PF)