In Metroid Prime 2: Echoes, a Guardian is the Darkling boss of an area and guards its Dark Temple containing a certain amount of planetary energy. There are three Guardians, in Dark Agon Wastes, Dark Torvus Bog, and the Ing Hive:
Although the Emperor Ing is not specifically considered a Guardian, it serves the same in-game purpose as the other Guardians (protecting its respective Dark Temple and planetary energy).
Interesting to note is that Samus had seven of her upgrades stolen by Warrior Ing, but only the six listed above are encountered. The Sub-Guardian with the Missile Launcher was never known to be seen, but the Missile Expansion dropped by the Dark Missile Trooper resembles Samus' Launcher, implying the Ing that stole it also possessed this Trooper.