Some commentators would suggest that John is here foreshadowing his later comment that the “armies of heaven” were riding “horses” as they fought alongside Jesus (Revelation 19:14, Revelation 19:19). The present paragraph, however, contains no allusion to war, and so using Revelation 19:14 and Revelation 19:19 to interpret the horse's bridle seems farfetched. It would be better to understand the reference as hyperbole through which John (and the Spirit) uses graphic exaggeration to communicate the huge amount of blood that flows from the outpouring of God’s wrath.
20 And the winepress was trodden outside the city, and blood flowed from the winepress, as high as a horse’s bridle, for 1,600 stadia.