Since the last half of the book is really an A-Z guide to fantastic beasts, the book really just ends at the end of the alphabet. So, we stop at the last entry—the Yeti—and that's it.
Why is that the end? Well, why is zyzzyva (a genus of South American weevils) the last word in the dictionary? Because it is. Because there are no more words after it.
Yeti is the last fantastic beast in alphabetical order and so it's the last one we see in our alphabetical guide to all the known magical creatures.
But we'll point out that we can't help but feel a little cheated that J.K. Rowling couldn't think of at least one creature with name that started with the letter Z. After all, we were promised "An A-Z of Fantastic Beasts" right? We got an A-Y. Not cool. Hey, even Dr. Seuss brought out the Zizzer-Zazzer-Zuzz.
Looks like Mr. Scamander needs to revise for a 53rd edition.