Who authored the Mevo Hatalmud? It is commonly assumed to have been authored by R. Shmuel Hanagid, and this is assumed by R. Ishtori Haparchi in his Kaftor Vafreach (ch. 5) and by the Chida in Shem Hag'dolim, but this page merely says that he is "generally believed to be the author". I have heard that it was actually authored by Sh'muel Bar Chofni.
So, who wrote it?
A source earlier than R. Ishtori Haparchi stating that it was authored by R. Shmuel Hanagid would suffice for an answer. (Although I would prefer a more thorough discussion of the matter rather than a mere assumption.)