Domain Zoo is a fast aggro-midrange deck that races hard, so this matchup is mostly won or lost on tempo rather than sideboard tech. Stubborn Denial is a cheap way to protect a key threat like an equipped Puresteel Paladin or Stoneforge Mystic from their removal (Lightning Bolt, Tribal Flames) or counter one of their own spells, and with a 4-power creature in play Ferocious makes it a hard counter. We only make this one small swap since our deck's game plan already lines up well against their lack of targeted hate for artifacts; keep the full equipment suite intact to maximize the speed of our clock.