Horayos seems to be laid out differently from the usual--with Rashi (or what is called Rashi, even if perhaps not by him) on both sides, Tosafos at the bottom and dedicated space for Rabbeinu Chananel on every daf. Does anybody know why? Thanks.
This is because when the pagination of the Talmud was standardized, no Tosafot were printed on the page for Horayot.
Here's the first page of the Venice edition of the Horayot that standardized the pagination of the Talmud:
As you can see, it has Rashi but no Tosafot.
The Tosafot were added in the Vilna edition with this note to explain their provenance: They were found in a manuscript and had already been printed with Hagahot Asheri on Sukka.
Since the form of the page had already been standardized, they were added outside the page along with the other things added in the Vilna edition (such as Rabbeinu Chanan'el, which was also printed for the first time in the Vilna edition).