Why does Hashem not desire the horse's strength?
Radak: He gave it to its strength - "ha'Siten la'Sus Gevurah" (Iyov 39:19; will you give strength to the horse, like I did?!)
Malbim: Some get their food via war; they take spoils from the enemy. 'They came to David, and said that Yisrael needs income; he told them to go to war' (Brachos 3b). Horses are needed for war.
What is the meaning of "He does not desire man's thighs"?
Rashi: He does not desire man's ability to run quickly.
Radak: He does not desire one who trusts in his thighs to flee from war - "b'Raglav Lo Yemalet" (Amos 2:15).
Malbim: Some get their food via going to distant lands - "mi'Merchak Tavi Lachmah" (Mishlei 31:14). They need [strong] thighs and feet to go from place to place.
