Confidence:StrongSolidPlausibleTentativeSpeculative

• Intruder Alarm

  2U

  EnchantmentU

  One-directional refresh: one card can untap permanents the other wants to reuse. Not self-sustaining on its own — the untapper has to keep showing up.
• Sky Hussar

  3WU

  Creature — Human KnightUW

  One-directional refresh: one card can untap permanents the other wants to reuse. Not self-sustaining on its own — the untapper has to keep showing up.
• Aura of Dominion

  UU

  Enchantment — AuraU

  One-directional refresh: one card can untap permanents the other wants to reuse. Not self-sustaining on its own — the untapper has to keep showing up.
• Baron Helmut Zemo

  BBB

  Legendary Creature — Human Noble VillainB

  Copy engine: one card makes a copy of a creature every turn, and the other's enter-the-battlefield effect rewards each copy. Real, repeatable value — the partner can't untap or re-enter the copier, so it never goes infinite.
• Copperhorn Scout

  G

  Creature — Elf ScoutG

  Copperhorn Scout untaps all (or many) permanents, refreshing Hellspur Posse Boss's tap activation.
• White Plume Adventurer

  2W

  Creature — Orc ClericW

  White Plume Adventurer untaps all (or many) permanents, refreshing Hellspur Posse Boss's tap activation.
• Jorn, God of Winter // Kaldring, the Rimestaff

  2G1UB

  Legendary Snow Creature — God // Legendary Snow ArtifactBGU

  Jorn, God of Winter untaps all (or many) permanents, refreshing Hellspur Posse Boss's tap activation.
• Jace, Ingenious Mind-Mage

  4UU

  Legendary Planeswalker — JaceU

  Jace, Ingenious Mind-Mage untaps all (or many) permanents, refreshing Hellspur Posse Boss's tap activation.
• Selvala, Eager Trailblazer

  2GW

  Legendary Creature — Elf ScoutGW

  Both cards produce the same kind of effect (shared token subtype).
• Aggravated Assault

  2R

  EnchantmentR

  Aggravated Assault untaps all (or many) permanents, refreshing Hellspur Posse Boss's tap activation.
• The Thirteenth Doctor

  1GU

  Legendary Creature — Time Lord DoctorGU

  The Thirteenth Doctor untaps all (or many) permanents, refreshing Hellspur Posse Boss's tap activation.
• Veteran Beastrider

  1GW

  Creature — Human KnightGW

  Veteran Beastrider untaps all (or many) permanents, refreshing Hellspur Posse Boss's tap activation.
• Mirrorhall Mimic // Ghastly Mimicry

  3U

  Creature — Spirit // Enchantment — AuraU

  One-shot copy enabler: a card enters as a copy of another (Phantasmal Image, Body Double, Phyrexian Metamorph), firing the partner's substantive ETB once. Not a repeating chain — fires once on entry — but valuable when the partner's ETB is combo-grade (graveyard reanimator, mana doubler, etc.).
• Ertha Jo, Frontier Mentor

  2RW

  Legendary Creature — Kor AdvisorRW

  Both cards produce the same kind of effect (shared token subtype).
• Drumbellower

  2W

  Creature — SpiritW

  Drumbellower untaps all (or many) permanents, refreshing Hellspur Posse Boss's tap activation.
• Gideon, Martial Paragon

  4W

  Legendary Planeswalker — GideonW

  Gideon, Martial Paragon untaps all (or many) permanents, refreshing Hellspur Posse Boss's tap activation.
• Dazzling Theater // Prop Room

  3W2W

  Enchantment — Room // Enchantment — RoomW

  Dazzling Theater untaps all (or many) permanents, refreshing Hellspur Posse Boss's tap activation.
• Intellectual Offering

  4U

  InstantU

  Intellectual Offering untaps all (or many) permanents, refreshing Hellspur Posse Boss's tap activation.
• Dino DNA

  1

  ArtifactC

  Copy engine: one card makes a copy of a creature every turn, and the other's enter-the-battlefield effect rewards each copy. Real, repeatable value — the partner can't untap or re-enter the copier, so it never goes infinite.
• Nezumi Linkbreaker

  B

  Creature — Rat WarlockB

  Both cards produce the same kind of effect (shared token subtype).
• Prophet of Kruphix

  3GU

  Creature — Human WizardGU

  Prophet of Kruphix untaps all (or many) permanents, refreshing Hellspur Posse Boss's tap activation.
• Éomer, Marshal of Rohan

  2RR

  Legendary Creature — Human KnightR

  Éomer, Marshal of Rohan untaps all (or many) permanents, refreshing Hellspur Posse Boss's tap activation.
• Cursed Mirror

  2R

  ArtifactR

  One-shot copy enabler: a card enters as a copy of another (Phantasmal Image, Body Double, Phyrexian Metamorph), firing the partner's substantive ETB once. Not a repeating chain — fires once on entry — but valuable when the partner's ETB is combo-grade (graveyard reanimator, mana doubler, etc.).
• Ivorytusk Fortress

  2WBG

  Creature — ElephantBGW

  Ivorytusk Fortress untaps all (or many) permanents, refreshing Hellspur Posse Boss's tap activation.
• Unstoppable Plan

  2U

  EnchantmentU

  Unstoppable Plan untaps all (or many) permanents, refreshing Hellspur Posse Boss's tap activation.