As you mow your way through LA’s zombie hordes, each type you defeat will be added to your Zompedia. There are regular zombies and Apex Variants, and by the penultimate Story Quest titled ‘End of the Line’, you’ll encounter a Putrified Slobber, which will likely be the last entry you’re missing.

If the achievement hasn’t popped by then, keep killing zombies until you scratch all types off your list. You may find certain types in different places, but during the course of the story, you’re pretty much guaranteed to encounter them all at some point.

Check out the full list of Zombie types, Apex Variants, and their locations through the link.