Some have said that the owner of the animal has more right to sit in front on the animal. However, if he gives permission to someone else (to sit in front), then it is permissible.

Narrated Aiyub: The worst of three (persons riding one, animal) was mentioned in `Ikrima's presence `Ikrima said, "Ibn `Abbas said, '(In the year of the conquest of Mecca) the Prophet came and mounted Qutham in front of him and Al-Fadl behind him, or Qutham behind him and Al-Fadl in front of him.' Now which of them was the worst off and which was the best?"