Paul at the Blog of Holding has put together a fantastic one page index of monsters [PDF] for the Dungeons & Dragons 5th Edition Monster Manual, listing them not only by CR level, but also detailing what page they can be found on. This builds upon not only the official one released by Wizards of the Coast as a PDF, but also the one developed by Mike Shea at Sly Flourish [PDF], and allows you to put it in inside the book, or perhaps in your campaign folder.
This table may be included in the Dungeon Master’s Guide that will be releasing later in the year. While a few posts have brought up the the fact that this table has been missing such as at Critical Hits and that others have stepped in to create tools before the release of the PDF , the omission of such an index from the actual sourcebook for monsters is a significant one. This is especially true in light of the fact that the new version of the game is trying to make itself much more accessible to new players of the brand, and another piece of information given in a box in the free Dungeon Master’s Basic Rules Book on page 56 (of v1.0):
Much of the advice in this section focuses on the XP values of monsters and encounters, as opposed to their challenge rating. Challenge rating is only a guidepost that indicates at what level that monster becomes an appropriate challenge. When putting together an encounter or adventure, especially at lower levels, exercise caution when using monsters whose challenge rating is higher than the party’s level. Such a creature might deal enough damage with a single action to overwhelm characters of a lower level. Even though an ogre has a challenge rating of 2, for example, it can kill a 1st-level wizard outright with a single blow.
For the beginning DM, knowing what monsters to choose to present to your party isn’t easy when confronted with a book like the Monster Manual and some sort of index included in it to point to them. Not including an index like this in the book seems like a major oversight and may turn off potential purchasers of the product, who may feel that they now absolutely need to buy the third book in the set in order to use the first two properly.