In the books
In the A Song of Ice and Fire novels, Cragorn is a crewman on the Silence, noted for having a large tattoo of a bird of prey on his chest. It is unknown if Euron ripped out his tongue, as he does for all of his crew. It is also unclear if he is an ironborn or is one of the foreign sailors that Euron picked up during one the many years he was raiding distant lands.
While Cragorn and the horn didn't appear in the TV version of the Kingsmoot, in a subsequent interview actor Pilou Asbæk (Euron) heavily implied that this was simply pushed back to appear later in the TV show.