Whether the Prohibition against Paining Animals is Rabbinic or Biblical: Shabbat 128b, 154b; Bava Metzia 32b-33a
Using a clever trick to prevent violation of Prohibition against Paining Animals: Shabbat 117b
Prohibition against Paining Animals vs. Prohibition against moving certain items on Shabbat/Holidays: Shabbat 128b
Prohibition against Paining animals as the possible cause for the Mitzvah of helping unload an Animal: Bava Metzia 31a, 32a