Hmmm...all I can say in defence of my reading of that line is that Carthy never uses Scots pronounciations (he feels that it's inappropriate unless it's your natural accent) and so would have sung "go" instead of "gae", and there's no mention of a "coal-black" horse anywhere in the song: I've never come across "coal" used on its own in reference to horse-colour.  And, of course, the fact that I sincerely believe myself to be right! ;-)

Malcolm