Note: this is a visual bug only, the extra pets dont do anything. I was doing arachnaphobia and accidentally left my pet "on guard" in the dungeon when I finished out. When i went back in and summoned a new pet, I turned the corner and was suprised to find my old pet still at its post. I now had 2 pets there. Repeated this and left my 2nd pet there and came in with a 3rd pet, to find both of my old pets on guard.
Don't know if anyone else doing the quest will run into the pets or not, but it seems this could clog up resources, causing lag and locking out instances. Screenshot of me and 3 pets enclosed