John Calvin Commentary Genesis 49:14

"Issachar is a strong ass, Couching down between the sheepfolds:" — Genesis 49:14 (ASV)

Issachar. Here, mention is partly made of the inheritance, and an indication is partly given of the future condition of this tribe. Although he is called a bony ass on account of his strength, which would enable him to endure labors, especially rural ones, yet at the same time his sloth is indicated: for it is added a little later that he would be of a servile disposition.

Therefore, the meaning is that the sons of Issachar, although possessing strength, were nevertheless quiet rather than courageous, and were as ready to bear the burden of servitude as mules are to submit their backs to the packsaddle and the load. The reason given is that, being content with their fertile and pleasant country, they do not refuse to pay tribute to their neighbors, provided they can enjoy repose.

And although this submissiveness is not publicly mentioned either to their praise or their condemnation, it is nevertheless probable that their indolence is censured, because their lack of energy prevented them from remaining in possession of that liberty which had been divinely granted to them.