Rashi on Genesis 49:14/רש"י על בראשית מ״ט:י״ד

1יששכר 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).

2רבץ בין המשפתים CROUCHING BETWEEN THE FOLDS — Like an ass that travels day and night and has no permanent stable of its own, so that when it wishes to rest it croucheth down between the borders — i.e. at the bounds of the cities whither it is earring merchandise .