The Ancient Lion Knight's spawn itself has nothing special. It respawns every 3-5 minutes depending on the number of online players. Nothing makes it disappear besides, of course, it walking away from its spawn home (which is a particularly small area for the Lion Knight).
The Usurpers there, however, do have some special mechanic. Whenever someone enter Drume's area, many Usurpers around Bounac will "disappear" because, in fact, they have moved to the fight to aid Drume against Kesar and the player(s) there. After this battle ends, Usurpers will start respawning.