Interestingly, the Rambam says that only a ram’s horn is valid for a Shofar. The HIDA (Birke Yosef § 586) explains that if one does not have access to a ram’s horn, one should not make a blessing over other species of Shofar. For example the horn commonly known as the Yemenite Shofar is a long ibex’s horn. Rabbi Ben Zion Abba Shaul (Or LeTzion vol. 4) says that if a Yemenite Shofar will be used, that at least the first thirty blasts should be from a ram’s horn.
Summary: The ideal way to fulfill the Mitzvah of listening to the Shofar is with a ram’s horn.