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. One day he and a little friend were playing with the donkey, trying to make him move and pull the cart.