Guardian
"Guardian" (ガーディアン Gādian) is an archetype of monsters used by Rafael, Apollo and Diana in the anime. The original "Guardian" monsters each have a corresponding Equip Spell Card.
"Celtic Guardian", "Winged Dragon, Guardian of the Fortress #1", "Winged Dragon, Guardian of the Fortress #2", "Guardian of the Labyrinth" and "The Reliable Guardian" are not "Guardian" cards as they do not include 「ガーディアン」 in their Japanese names. As such, the only "Guardian" support card in the TCG/OCG, "Arsenal Summoner", specifically excludes them.
The core "Guardian" monsters were initially released in Dark Crisis and re-released in Dark Revelation Volume 1. They can only be Summoned if the player already controls their corresponding Equip Spell Card. However, they can still be Set without controlling their respective Equip Spell Card. The Equip Spell does not have to be equipped to their corresponding monster, and due to their Summoning conditions, usually cannot be. "Guardian Eatos" is the only one of the "Guardians" with an associated Equip Spell Card who can be Summoned without restrictions.