Doc Q shares several similarities to the fourth horseman, “Death”, of the biblical figures “The Four Horsemen of the Apocalypse” mentioned in Christianity. In many illustrations of what “Death” may look like, he is described as a gaunt figure who rides a sickly and pale (or gray) horse while wielding a scythe much like the grim reaper. Doc Q weilds a scythe.
And his epitaph is “Death God”. And horseman of death is commonly portrayed as either a gaunt man on a pale horse or a skeleton and Brook is not only a skeleton but he is someone who died but came back to life so it would fit for Brook to fight someone who alludes to the horseman of death for by being brought back to life he defied death in a sense.
*Theory by shadow22