The Antlion Guardian (referred to as "Ancient Guardian" by the Vortigaunt) is a variant of the Antlion Guard encountered in Half-Life 2: Episode Two. They appear physically identical to Guards, but have a different coloration and are bioluminescent.
Apparently, each Antlion hive has one Guardian that lurks near the larval clusters. The Guardian patrols the "Nectarium" (center of the nest), keeping watch for nest raiders. Guardians are extremely protective of Antlion Grubs, the extract of which is used by Vortigaunts to manipulate the Vortessence, and will ferociously attack intruders in order to protect them.
Should someone enter the "Nectarium" for larval extract, the Guardian will attack and may pursue the thieves for some distance (in Episode Two, a Guardian follows Gordon Freeman (whose Vortigaunt companion had taken the extract outside) and battles him).
It is notable that the Guardian is very similar to the Guard, both in name and appearance, but there are numerous differences: the Guardian often lives deeper within the nests, protecting and ruling over the Antlions and eggs within. Comparatively, the Guard will work closer to the surface, protecting nest entrances, and offering backup for workers. Also notable is the Guardian's natural bioluminescence, which is presumably caused by it living in deeper darkness. The Guardian also has a very different coloration, consisting of more varied and contrasting colours.