Because Lovecraft had never seen a copy of Regnum Congo when he wrote "Cool Air", or perhaps just because he was taking dramatic license, his description of the book is not terribly accurate — which is ironic considering that he knocks Theodor de Bry for basing his engravings on poor second-hand descriptions. For one thing, all the plates were at the back of the book, and there were only fourteen of them, so Plate XII is actually almost at the very end, not in the middle as described. Here's a look at some of the other plates that the creepy old man enjoys looking at.
Here is Plate IX: a zebra as drawn by someone who has never seen one, but to whom it has apparently been described as like a horse but with stripes.
Here is Plate XI: featuring a tree with "big leaves a-floppin' over and daown", and a creature like "a dragon with the head of an alligator", and a person that the ignorant old man might mistake for an "Injun".
You can see all the other plates and read Regnum Congo at Google Books.