Guardian

Final Doom (PSX)
The Guardian is final boss of Plutonia on PSX Final Doom, it is a Cyberdemon  It is found in MAP30: Onslaught (The Plutonia Experiment).

Doom 3
The Guardian of Hell is a boss in Doom 3.

Doom Eternal

 * Doom Hunter (priests' guardian)

Doom (2016)
The Guardian (possibly a reference to the Doom 3 Guardian) defeated the Great Serpent, and enslaved the Hell Knights.

Hexen

 * Guardian of Ice
 * Guardian of Fire
 * Guardian of Steel

Orcs & Elves
An enemy class found in Orcs & Elves series.
 * Guardian
 * Shadow Guardian