Guardian
Guardian is an obsolete creature class. Guardians are creatures which guard, protect, or preserve.
The first creature to bear the type was Guardian Beast from Antiquities. Other examples include Harbor Guardian and Fountain Watch. The subtype was obsoleted in the Grand Creature Type Update. Ivory Guardians used to give a bonus to all Guardians. It now gives the bonus to just "creatures named Ivory Guardians".
On Ravnica, the Simic Hybrids are also known as "Guardians".