I have absolutely no idea how this happened. I sent three of my units out on an errand to a volcano to look for a gold vein and they come back with TEN chocobos and an egg.
 
Now I don't mind one or two monsters tagging along...but TEN?! Why did this happen? I've now dismissed all but Boco and a Black Chocobo. I'm hoping that the egg will be a Red Chocobo so I can just dismiss any that wish to join in the future.