Issachar is a strong ass couching down between two burdens:

Issachar is a strong donkey, lying down between the sheepfolds.

And he saw that rest was good, and the land that it was pleasant; and bowed his shoulder to bear, and became a servant unto tribute.

He saw that his resting place was good and that his land was pleasant, so he bent his shoulder to the burden and submitted to labor as a servant.