If a monster is equipped to a monster like "Relinquished" as an Equip Card, then the monster is not treated as a Monster Card. Thus, you cannot Special Summon "Exxod, Master of The Guard" by Tributing a "Sphinx" monster that is being treated as an Equip Card and is equipped to another monster.[2]
References
↑ abKonami FAQ: Effect Monster > Exxod, Master of The Guard
↑Konami FAQ: Can you Special Summon "Exxod, Master of The Guard" by Tributing a "Sphinx" monster that is treated as an Equip Card?