Elephant tusks are solid ivory over almost their whole length. This looks to be hollow and the fact you say it has been made into a horn re-inforces this. The outer sheath of a 'cow horn' looks exactly like the item here. The centre of a Cow horn is made of bone and is part of the skull. The central bone is covered with a Keratinous sheath which is hollow and can be made into an instrument which makes a sound when blown - a 'horn'.
The writing looks to be written in the style of roman numerals, MDIX would be 1509, how old could this be?