They were used as blood sacrifices, but the donkey, to be of any use, must be born into immediate redemption by lamb's-blood. Rebellion comes from stubbornness and that is under Christ's vast forgiveness. The blood of the Lamb covering our basic sin identity - that of a donkey. Sheep we are. Sheep are vulnerable and dumb.
Sheep stands for humanity.
Christ Himself became sheep like us. . . As humanity, sheep are merely weak, but donkey is our sin nature whose secret root is a set and unyielding heart. Sheep are dumb. Sheep are believers. Goats are foolish.