Several guards were present in the Vanguard Outpost, though it's unknown what sort of role or AI they may have had. The Sentinel Guard in Avant Gardens largely used the same design as these early guards, though the pipeline for creating NPCs had changed by then and the old model was no longer used.
On December 19th, 2011, a Mythran named Shival hosted a party for players in Nexus Tower, spawning various NPCs and creatures, including the Sentinel Guard.
On January 30th, 2012, a Mythran mailed the Sentinel Guard's shield to players. Strangely, it occupied the player's right hand slot even though the Sentinel guard holds it with his left hand.