Michael Chesid in the Shofar blog makes a point about the use of horn from non-kosher animals:

 

While an individual may prefer to use a Shofar made horn the horn of an animal slaughter slaughtered in accordance with Jewish dietary laws of kashrut, this is not a requirement according to most interpretations of Jewish law. The Shofar, after all, is neither placed inside the mouth when blown nor eaten.

 Beyond the dietary laws is a principle that religious articles must be made from sources that are muttar be-fikha (literally: permissible in your mouth), a precept derived from the commandment regarding tefillin in Exodus 13:9, "in order that the Teaching of the Lord may be in your mouth." A careful analysis of this principle limits its applicability to sacred writing. [1] This precept, moreover, is countered by the concepts that items which are afra be-alma (mere dust), including items that are inedible from their inception, are not considered not kosher to eat. More, non-kosher animal products besides a Shofar horn are allowed in ritual use, including dye from a mollusk and silk for tzitzit and an elephant as a wall for a sukkah. [2] Instead, Shofars are tashmishei mitzvah, [3] objects used to perform a mitzvah yet do not have inherent kedushah (holiness).

The use of horns from non-kosher animals is reflected in the commercial trade in shofarot, much of which is based upon horns from animals slaughtered in North Africa and other countries without an abundance of ritual slaughterers. Many of the long spiral horns of the Kudu are taken by hunting, a non-kosher means of slaughter, as most likely were the horns of mountain goats referenced in the Talmud ##verify

This is good news for individuals who have ethical objections to the slaughter of animals since it allows the use of horns from animals that die from natural deaths or accidents instead of being ritually slaughtered.

[1] Mois Navon, "The Hillazon and the Principle of 'Mutter be-Fikha'", The Torah u-Madda Journal, October 2001, www.yutorah.org/_shiurim/%2FTU10_Navon.pdf, March 4, 2006. This article is also the source for other information in this paragraph.

[2] Sukkah 23a

[3] According to Megillah 26b, "These are tashmishei mitzvah: sukkah, lulav, Shofar, tzitzit."

Mr. Chesed brings up a valid point; the issue becomes: Can a Shofar be made of a non-kosher animal?

 

The Mishnah Breuah states that the best type of Shofar is made from a ram and is bent (MB 586:1). Further, on, the Chofetz Chaim posits that the best type of Shofar should be made from a ram whose horn is bent; the middle type, a kosher animal; the least, any kind of animal. (MB 586:16). Note that there is no requirement for ritual slaughter (shechitah).

 

Although some disagree with this priority, the horn can be made form a non-kosher animal on the basis of Mutar B’ficha (the material is good enough to put into your mouth). Moreover, since the principle halacha is to hear the Shofar, the Rabbis use the [principle of tashmishe mitzvah (for things that surround the mitzvah, in this case the original of the hollow horn, do not have to be strict).  (MB 586:16 (8).  Thus, even nevaylah or treif is all right.  However, the Magen Avraham opines a stricter standard by saying that all has to be pure.

 

In addition, the interpretation form Avot 67b calls for Non-kosher “food” unfit for human consumption is not called food. Thus, the Tannah. R. Meir poskins that animal hairs, anointing oil and incense (animal secretions) and dyes of crimson (royal or holy dyes made from mollusks, a treif food) are acceptable (Megillah 26b)

 

In addition, the Elef haMagen states a priority of Shofar use:

 

The Elef Hamagan (586:5) delineates the order of preference: ¹⁵ 1) curved ram; 2) curved other sheep; 3) curved other animal; 4) straight - ram or otherwise;¹⁶ 5) non-kosher animal; 6) cow horn. The first four categories are used with a beracha, the fifth without a beracha, and the final, not at all. The Mishnah Berurah (586:4) subdivides level three with a preference for a goat . . .

(See  Elef Hamagen , Rabbi Shemarya Hakreti Author: Erand Aharon Editor Jerusalem: Mekitzei Nirdamim, 2003).