The "current" list is arranged by exhibit or area, and there is a section he has titled "along the way" for random enclosures. The "former" list is also arranged by exhibit, including the Small Mammal House, but also has an "along the way" list which doesn't really make sense in the context of the "current" list's arrangement - so I think he might have been using that part for former species he didn't know where to place.
That's what I took it as for the former list, animals that were kept but no source mentioned their location within the zoo, or their exhibit no longer exists.