Rashi on Genesis
יששכר חמר גרם means ISSACHAR IS A BONY ASS — he bears the yoke of the Torah (i.e. obligations) like a strong ass upon which may be placed a heavy load (Genesis Rabbah 99:9).
Rashbam on Genesis
יששכר חמור גרם, he is not like Zevulun who engages in voyages with mariners to distant lands to engage in trade and commerce. He stays at home toiling his earth sharing the fate of his donkeys, his beasts of burden.
Sforno on Genesis
יששכר חמור, not ready to fight a war. This corresponds to the saying of the sages in Avodah Zarah 17 אי ספרא לא סייפא, “if your vocation is books, it cannot be skill with a sword.”