Pets. Well, it's an interesting thing. Basically, as you may notice by playing on prod, the pet moves basing on the player's movement. But as you can easily see, it doesn't stand on the player, and doesn't move for every single player's movement. This is called (I think) "movement relevance calculation", and consists in a piece of code that checks the player's distance from the pet and if it's farther than a specified value, the pet will move to that location.
Regarding what pets can do, well, it's wonderful. As they are entities, we can make them friendly and still use the enemy behaviors code. This opens for a bunch of ideas and possibilities: imagine a special pet, which within a certain situation orbits the player and absorbs some damage/heals him, or enemy pets too. We could even implement PvP, yeah.
So much code, so many ideas, so many possibilities and purposes, yet so little time.