Just in time for Daf Yomi, R. Natan Slifkin posted an extensive essay titled "Exotic Shofars: Halachic Considerations" (link - PDF). Here are the section headings:
- Kosher Animals
- Hollow Horns and the Tzvi
- The Requirement of Curvature
- Straight vs. Curved
- The Problem with Ordinary Shofars
- The Preference for Rams
- Requirement vs. Preference
- The Yemenite Kudu Shofar
- The Problem with Gemsbock
- The Gemsbock as the Re'em
- The Keren of the Re'em
- An Argument for Permissibility Summary