The Gemara quotes a baraita where we learn that two people pulling (moshkhim) a camel or directing (manhigim) a donkey can claim that they own the animal. Similarly, if one was pulling and the other directing, their claims would be accepted. Rabbi Yehuda disagrees, arguing that for a camel only meshikhah will be significant while for a donkey only manhigah would be significant.
From this discussion the Gemara suggests that we can conclude that riding the animal will not be significant in either case, since the possibility of riding is not raised at all. In response the Gemara says that the situations that appear in the baraita may only be those in which there is a disagreement between the Tanna Kamma and Rabbi Yehuda, but in a case where all agree – i.e. if the person is riding the animal – there is no need to mention it in the baraita at all.
The difference between manhigim – directing a donkey – and moshkhim – pulling a camel – stems from the animals’ respective temperament and nature as well as from the way these animals were trained. A donkey is “directed” in the sense the donkey walks ahead, leaving the person in charge (the hamar) walking behind the animal, holding a stick with which he directs the animal. This would not work with a camel, so the common practice was to muzzle the animal and have the person in charge (the gamal) walking in front and pulling the animal which follows him. The difference in the way each of these animals was treated was so stark, that an expression used by the Sages to indicate an internal contradiction was hamar-gamal, meaning a person who must play two contradictory roles simultaneously.