This is a reference to the disagreements of the early jurists and various schools on the question of liability if one's riding animal hits another person. Hossein Modarressi cites this source to present a comparison between these views and that of Jaʿfar al-Ṣādiq, which restricted liability to the case of the animal hitting another person with its forelegs, not hind legs.
This source is part of the Online Companion to Hossein Modarressi, Text and Interpretation: Imam Jaʿfar al-Ṣādiq and His Legacy in Islamic Law (Harvard University Press, 2022).