The virtue of piety also requires that one should not cause any creature to suffer, even animals, and should show mercy and compassion toward them. Similarly, it says (Proverbs 12:10): ‘The righteous man knows the soul of his beast.’ And there are those who are of the opinion that ‘causing an animal to suffer is a prohibition of the Torah’ (Shabbos 128B). And at the very least it is a Rabbinic enactment.
-Rabbi Moshe Chaim Luzzatto, The Path of the Just, Ch. 19