An oliphant is an ivory horn. (The oliphants linked below are identified by their museum accession numbers.) Most of the examples I've found were made in southern Italy in the 11th or 12th centuries, and appear with different styles of carving around the surface of the ivory; some later examples have little or no surface carving.

Perhaps the most famous oliphant in medieval literature is Roland's:

Rollant ad mis l'olifan a sa buche, Empeint le ben, par grant vertut le sunet. Halt sunt li pui e la voiz est mult lunge, Granz .XXX. liwes l'oïrent il respundre.

Roland hath set the olifant to his mouth, He grasps it well, and with great virtue sounds. High are those peaks, afar it rings and loud, Thirty great leagues they hear its echoes mount.

The Grove Dictionary of Art suggests that Roland’s oliphant “may have contributed to, or at least reflected, the widespread popularity of this kind of horn as the hero’s horn par excellence.”

• Musée départemental des Antiquités de Rouen inv. 1796, Salerno or Sicily, 11th century
• Musée de l'Arles et de la Provence Antiques, 11th century
• The Horn of Ulph, made in southern Italy in the 11th century
• Schatzkammer of Aachen Cathedral, 11th century
• British Museum OA+.1302, 11th-12th century
• Louvre OA4069, Amalfi (?), 11th century (back, detail)
• British Museum M&ME 1923,12-5,3 (The Borradaile Oliphant), Byzantine (made in southern Italy), 11th century
• British Museum M&ME 1979,7-1,1 (The Clephane Horn), southern Italy, 11th century
• V&A 7953-1862, Amalfi or Salerno, 11th century
• Kunsthistorisches Museum Wien 4073, southern Italy, 11th century (with later inscription indicating that it was a gift from Count Albrecht III von Habsburg to Kloster Muri in 1199)
• Oliphant, Byzantine, late 11th century; probably from a cathedral treasure, mentioned in 17th century inventories
• Louvre OA 4069, southern Italy, late 11th century
• Louvre MRR 430, southern Italy, late 11th century
• Musée Paul Dupuy (the oliphant known as the Horn of Roland), Salerno or Amalfi, end of the 11th century
• Swedish Museum of National Antiquities (Historiska Museet) 289, southern Italy or Sicily, 11th-12th century
• Museum für Islamische Kunst K 3106, Sicily or southern Italy, 11th-12th century
• Sotheby's Sale L08233, Lot 5, Sicily, c. 1100
• MFA 57.581, southern Italy (probably Amalfi), c. 1100
• Musée national du Moyen Âge Cl.13065, from the Abbey of Saint-Arnoul de Metz, made in southern Italy c. 1100: left, right, front, back
• Museo de Pilar (the Oliphant of Gaston IV, Viscount of Bearn)
• Metropolitan Museum of Art 17.190.218, southern Italy, 12th century
• Cleveland Museum of Art 1930.740 (the Horn of Saint Blaise), Palermo, 12th century
• Musée national du Moyen Âge MR360, southern Italy, 12th century
• Musée de l'Armée P 575, 13th century (detail)
• Fundación Lázaro Galdiano 2577, Spain (possibly Cordova), 13th-14th century
• Openbaar Centrum voor Maatschappelijk Welzijn, 15th century
• Museu Nacional de Arte Antiga, Sapi-Portuguese, 15th-16th centuries
• British Museum NN.25, made in Sierra Leone c. 1490-1530
• Musée national de la Renaissance ECL1859, Sapi-Portuguese
• Musée national de la Renaissance E.Cl.17079
• Musée national du Moyen Âge CL429, Brescia, 16th century
• Musée national de la Renaissance Cl.428, 16th century
• Cathedral of Santiago de Compostela
• Hunt Museum CG 046

Additional illustrations of hunting horns can be found in the hunting iconography linkspage.

• The fourth trumpet, Commentarius in Apocalypsin (BNF Latin 8878, fol. 141), before 1072
• Men blowing oliphants in the borders of a northern French bible (BNF Arsenal 592, fol. 14v), 11th century
• A stained-glass roundel from Sainte-Chapelle de Paris: an angel blows an oliphant at the resurrection of the dead, c. 1200
• The Resurrection of the Dead, relief fragments 1 and 2, from the left lintel of the tympanum of the central portal of the Last Judgment at the Cathedral of Notre Dame in Paris, c. 1220-1230
• Roland breaks his sword and sounds his horn, The Charlemagne Window at Chartres Cathedral, c. 1225
• The death of Roland, Chronicle of Rudolf von Ems (SBB Ms.germ. 623, fol. 22v), 13th-14th century
• Galahad the Strong and Simeu, History of the Holy Grail (BNF Fr. 105, fol. 120), first half of the 14th century
• Galahad the Strong and Simeu, History of the Holy Grail (BNF Fr. 9123, fol. 91), c. 1315-1335
• The Horn of Roland, Historia Karoli Magni (BNF Fr. 124, fol. 9v, details 1 and 2), 14th century
• Lancelot is the champion of Guinevere, Lancelot du Lac (BNF Arsenal 3479, fol. 610), c. 1405
• An angel from a fresco of Mary in an enclosed garden, at a church in Krašce, Slovenia, 15th century
• The death of Roland Grandes chroniques de France (BNF Fr. 6465, fol. 113), c. 1420
• Florimont begins to sound his horn, The Romance of Florimont (BNF Fr. 12566, fol. 37v), mid-15th century
• Merlin helps Arthur, The History of Merlin (BNF Fr. 91, fol. 173), c. 1480-1485
• Chroniques françaises of Guillaume Crétin, before 1515: Roland sounds his horn (fol. 124), Charlemagne weeps for Roland (fol. 133v), the funeral of Roland (fol. 141)