Sforno on Genesis 49:9/ספורנו על בראשית מ״ט:ט׳

1גור אריה יהודה, although Yehudah is not a fully grown lion, i.e. not enjoying the trappings of royalty, possessing a land, etc, he is at least comparable to a lion cub, preeminent among his brothers. In the future he will rule as king in the full meaning of the word. All this, in spite of the fact that

2מטרף בני עלית, you were spared by my son Joseph who did not kill you in his wrath although he hated you. This is the meaning of the words כרע רבץ כאריה, seeing that at that time he was like a crouching lion, one in repose, i.e. not representing a threat to anyone This is why Joseph did not issue you orders to kill you.

3וכלביא מי יקימנו, once Yehudah will attain the position of king, he will be like such a lion who on occasion will lie down languidly just like a lion, a time when no one can frighten him into rising because he has become afraid of the enemy.