"Big as a house, gray as a mouse," and very loud!

The Song of Roland is an old French epic poem in which the knights of Charlemagne battle Saracen forces.
The leading knight, Roland, has a horn that they refer to as an "olifant."

Olifant

 "Olifant" is the Old French word for "elephant." In fact, the present day English word is derived from the French (The American Heritage Dictionary). What is an elephants tusk? Ivory. Roland's horn was probably made of ivory.

In The Lord of the Rings, by J. R. R. Tolkien, there are some large elephant-like creatures called, "Oliphaunts."  (The title of this post comes from a poem about Oliphaunts in the book.)

Boromir also had an important horn.  It is mentioned in his first appearance in the book, "on a baldric he wore a great horn tipped in silver that now was laid upon his knees." He blew it as he defended Merry and Pippin from the Orcs, calling his companions: "Then suddenly with a deep-throated call a great horn blew, and the blasts of it smote the hills and echoed in the hollows, rising in a mighty shout above the roaring of the falls."  As Aragorn raced towards the scene, the horn sounded "desperately" and then ceased abruptly. He arrived too late: the Orcs had already run off with the hobbits. Aragorn found Boromir sitting against a tree, dieing. His horn, symbolic of his life, lay broken at his side. "But Aragorn saw that he was pierced with many black-feathered arrows; his sword was still in his hand, but it was broken near the hilt; his horn cloven in two was at his side. Many Orcs lay slain, piled all around him and at his feet."

As Roland fought the attacking Saracens, his friend Olivier told him to blow the horn and call for the help of Charlemagne's army.  Roland refused, and as a result the entire rear guard was wiped out, including himself and Olivier. However, before the battle was completely over, Roland blew the horn long and loud and called the army back after there was no more hope of survival. Before they arrived, Roland blew his horn again, but much weaker this time and Charlemagne knew that Roland was scarcely alive. Before he died, the warrior placed his sword and horn beneath him lest they fall into enemy hands. Charlemagne and his knights found the battlefield covered in bodies.