This document outlines what has changed between the 2014 and 2025 versions of the Monster Manual.
New Stat Blocks
The 2025 Monster Manual includes eighty-seven new stat blocks, including the Arch-hag, Blob of Annihilation, and Primeval Owlbear.
Revised Stat Blocks
Every monster in 2014 Monster Manual either appears in this book or has a CR-appropriate replacement (see below). All stat blocks carried forward from the 2014 Monster Manual have been revised with a focus on fun and usability.
Using This Book with Older Books
When you play D&D with the Monster Manual (2025), it replaces all monster stat blocks in the 2014 version of the book.
Fifty-nine stat blocks from the 2014 Monster Manual have name changes or CR-appropriate replacements. If you’re using material published prior to the 2025 Monster Manual and are unable to locate a stat block in it, consult the Stat Block Conversions table in appendix B of the 2025 Monster Manual to find the equivalent stat block you should use.

I'd say it's because gem dragons have already been redesigned.
Gem dragons have existed in previous editions, but didn't appear in 5e until the 45th anniversary of the game, when WotC released a statblock, miniature, and dice set for a revised adult sapphire dragon. Two years later, in 2021, they put the sapphire dragon and the other four main gem dragons into Fizban's, with updated designs.
It's also worth mentioning that the five main chromatic and five main metallic dragons have remained largely the same since 3e. Their designs haven't really changed all that much, until this new book.
All of this to say, it's the chromatic and metallic dragons that were due for a rework, not the gem dragons, since the gem dragons had just received one three years earlier.
This how it was in the 2014 MM for all core species. As more species are added to the core species, they also use the NPC stats, which is an advantage in the number of NPCs as stat blocks. Just add the species traits from the PHB and add what parts you need. If it is just a guard at the gate, maybe you don't worry about adding the species traits, because it is not going to be a combat encounter. Any species can be an opponent, so just add the species traits.
Just use the new stat blocks. If it is a CR6 creature in the adventure, the same creature in the new MM will still be CR6 just better.
Thank you for ruining it.
I am not sure if I am missing something but, apparently in Monster Manual 2025, Demons no longer can summon lesser demons as per page 53 (I think) of 2014 Monster Manual. The ability was not transfered to the monsters page and by removing page 50 to 54, that information is lost in the abyss. Sad isn't it. This reduces the potential of the Balor, Barlgura, Chasme, Glabrezu, Hezrou, Marilith, Nalfeshnee, Vrock and Yochlol. These sections of the 2014 Monster manual that give information about groups like those found in page 50 to 54, definately should have been preserved.
Summoning other demons is only part of what's been lost. I remember how terrifying Type VI Balors were way back in 1st edition because of how sophisticated they were in the many ways they could end you or interact with PCs. For all the editions since, other than some cool depth 2nd ed Planescape gave demons, the apex demon Balor has now been reduced in 5.5ed to "Hulk smash!" Other than a 60' teleport, it's just brute damage. Compare that to the awesomeness that was in 1st ed below.
Each type VI demon has its own name. (Balor is a type VI demon of the largest size.) Six are known to exist. The favored weapons of these monsters are a large + 1 sword and a whip with many "tails." The whip is employed to drag the opponent into the flames which the demons are able to create around themselves. During any combat there is a two-thirds chance (1-4 on a 6-sided die) each melee round that any type VI demon will immolate and use its whip. The two largest of these demons do 4-24 hit points of damage when a victim is drawn within their flames, the middle-sized and smallest do 3-18 and 2-12 respectively.
They shed darkness in a 10' radius ·at will. Their other singular abilities are: Cause fear (as a fear wand), detect magic, read magic, read languages, detect invisible obiects, cause pyrotechnics, dispel magic, suggestion, telekinese 6,000 gold piece weight, use a symbol of fear, discord, sleep or stunning, and they also have a 70% chance of successfully gating in a demon of type III (80%) or type IV (20%). Each of these terrible abilities can be employed as often as desired, but only one may be used at any given time.
With proper invoking, offerings, and promises type VI demons might be convinced to co-operate with a character or group for a time. Naturally, the demon will attempt to assume/usurp command at every opportunity. Most chaotic evil monsters are drawn to the strong evil charisma of this creature, and of all its kind, the type VI demon tends towards a more organized evil (which makes it less than popular with demon lords and princes).
All that plus old school 75% magic resistance that means 3/4 of the time they are immune to magic, and a TON of psionic abilities.
The new one? Just click the gamepad "hit" button as fast as you can to inflict lots of damage. That's it. Sigh....
Darth,
WOC has made it clear with their dumbass DEI crap that drow and orcs are a playable race, therefore, you will see no stat blocks like the old stats because, in there dumb ass view, calling orcs and drow evil is racist-which it isn't-its a flippin fantasy game.
Any one have a blank, fillable 2025 stat block template, I'll just convert them myself. No sense waiting on WOC's DEI thought police to they'll never update old monster stat blocks because its "racist".
I noticed a couple coding errors on the mobile app version of the book. There are a whole bunch of monster missing from the table of contents. All of the beasts, plus some odd ones too, like Faerie Dragons, Flameskull, Arcanaloth, Centaurs... They're all in the book, just not in the table of contents. If you search in the book, you don't get any results. But if you search everything, you can find them that way.
My best guess is that there's a missing heading tag for all of these creatures. Hopefully it is a quick fix as it makes it really annoying to find these in the app.
Here is the full list:
Can't believe werewolves don't have Damage immunities.
You know why they aren't? The same reason that Dragonhide belts don't work for the new Monk. So they can release the new Fizban's Treasury whenever they write it and get 60-80 more dollars out of us
Maybe for some reason you are only seeing the monsters in the free rules?
Nope. It's the full version that I paid for.
I found this thread after that has a bunch more info. It is a known problem.
https://www.dndbeyond.com/forums/d-d-beyond-general/bugs-support/215241-monster-manual-issues-and-support-thread?page=7#c146
There should be a way to toggle/switch to the 2024 version of monsters in adventure campaigns that are listed as "2024 compatible". If you choose to run one of these campaigns with 2024 characters, doesn't it make sense to allow for the updated creatures? I realize you can just search for the updated versions separately, but that negates the convenience of the in-text links already in place.
Man, I was so happy about the first two of the new core books (especially the amazing DMG!) and then this book just had to come along and sour the whole thing. Why did they get rid of so much of the lore? If this book is supposed to help newer DMs (as evidenced by the streamlining of casters and similar statblocks), then how does that decision make literally any sense? New DMs are exactly the group that needs this lore, as they're least likely to own previous monster manuals.
Sure, one can argue "but the books are intended to be setting agnostic, some people want to use different lore", but I still think the lore is important because not everyone has the worldbuilding experience and/or time to find a sensible place in the world (as well as things like typical culture, roles, personalities, goals, etc) for every single monster they might want to use at any point throught a campaign, and so the lore given in the book is supposed to serve as a ready-to-use thing you can freely ignore or change for your world if you want to. I just wish we could've had more lore instead of all this art (which is amazingly beautiful, don't get me wrong, but it actually helps you run a game, compared to the space it takes from the book, way less than pretty much literally anything else.