'Horn' in the Bible

I was in the Spirit on the Lord's day, and a great voice at my back, as of a horn, came to my ears,

After these things I saw a door open in heaven, and the first voice came to my ears, like the sound of a horn, saying, Come up here, and I will make clear to you the things which are to come.

Saying to the sixth angel who had the horn, Make free the four angels who are chained at the great river Euphrates.

But in the days of the voice of the seventh angel, when the sound of his horn is about to come, then will the secret of God be complete, of which he gave the good news to his servants the prophets.