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. Goats are unbelievers, but both were used for the sacrifice of the altar. Never was a donkey used for an offering. My father was an orphan, reared by a French family who adored him. Just for play, he was given his own donkey and cart.