Rhino is money (old slang). I'm a bit dubious about Silber's making quite so clear a link with the British Hunting the Wren, as it seems to be based on the existence of one single variant, found in America, into which Billy Barlow has somehow found his way; but perhaps there is more to it than that. On the other hand, Silber seems to have accepted at face value A. L. Lloyd's unsubstantiated assertion that the Wren songs have some link with the Peasants' Revolt, so I think we should be cautious about what he says on that score.