The Palace Guard is a Locust Theron Sentinel who has been selected as a bodyguard of the Locust Queen. While Theron Guards and Theron Sentinels represent elite soldiers of the Locust Hollow, Palace Guards are higher ranked than either Theron Sentinels or Theron Guards. Palace Guards are encountered only in Nexus.
Presumably, Palace Guards are selected from veteran Locust soldiers with outstanding service and dedication to the Locust Horde. The Triplex-Worm mark on the back of the some Palace Guards' armor indicate that they are trained by the Kantus, which would not only make Palace Guards the elite honor guards of the Locust Queen but also members of the Locust priesthood.