It would be an adjective, sort of. Thus "pombo do horizonte" would be something of a description (not a translation I mean), the imagery is that of a pigeon traveling and someone on ground seeing it free, free also to see the skyline, the remote lands, everything.
So, it can be seen also as the "pombo que vê o horizonte/que pertence ao horizone (que viaja, que tem o poder de ver longe etc).
I read somewhere else, that Elton John sung this song at Ryan White's funeral, the boy that was persecute and was at pains because he had AIDS, at the end the boy died of it. So, the lyrics might have meant that he was set free, that he was out of his pain of not being accepted and do what he wanted to do. He was free also of the problems associated with the disease, including prejudice, of course.
That said, and this way, the song would entail a sad lesson and tone to it, but on the other hand it would be of help in the sense of people be more understanding in the future.
Anyway, Elton John never said the song was to him, or inspired on that story. Perhaps it´s better to leave it at that, while people keep thinking how it come to be, they will remember what happened. Hence, a tool for change!
The song - translated.https://mais.uol.com.br/view/9lcpjrpj6p ... 6?types=A&