It's w not y.

src/engine/objects/Mob.java

@@ -378,7 +378,7 @@ public class Mob extends AbstractIntelligenceAgent {
             writer.putVector3f(mob.getLoc());
 
         //Rotation
-        float radians = (float) Math.asin(mob.getRot().y) * 2;
+        float radians = (float) Math.acos(mob.getRot().y) * 2;
 
         if (mob.building != null)
             if (mob.building.getBounds() != null && mob.building.getBounds().getQuaternion() != null)

src/engine/objects/NPC.java

@@ -646,7 +646,7 @@ public class NPC extends AbstractCharacter {
 			writer.putVector3f(npc.getLoc());
 
 		//Rotation
-		float radians = (float) Math.asin(npc.getRot().y) * 2;
+		float radians = (float) Math.acos(npc.getRot().y) * 2;
 
 		if (npc.building != null)
 			if (npc.building.getBounds() != null && npc.building.getBounds().getQuaternion() != null)