The assumption which states that “It’s a Boo-…” applies to Boots* seems to be plausible, but is based on written word “Boots”, still vocally “oo” means a long “u” and therefore in this case “Boots” has no priority over “Butcher”. It probably was indeed: “It’s a Bo-o-o-o-o-o-o-tcher!” I would rather believe in this interpretation, because in the poem, Butcher was described in great detail, and acted as one of the front characters, unlike Boots, who was just a “chorus member”. Actually, we even don’t know who Boots is at all.
The portrait of Butcher from the Oleg Lipchenko illustrations to the Hunting Of The Snark