Lots of monster types in this batch, but not that many monsters. The overwhelming majority of the mechanical changes in Monsters of the Multiverse went into humanoids and fiends; whether because they were designed and balanced better to begin with or because they just aren’t encountered as often, other monster types got away pretty clean.
On to fiends, which receive—by far—the greatest number of substantive changes in Monsters of the Multiverse, and that’s not even counting archdevils and demon lords. In fact, so many fiends receive significant updates to their actions that I’m going to break my examinations of this creature type into five posts: three for the rank and file (one each for the lawfuls, chaotics and neutrals) right now, then two more for the archfiends (one for archdevils, one for demon lords) after I’ve covered all the other creature types.
To begin with, the merregon’s Multiattack has been made unconditional: three Halberd attacks, period, whereas before it received the third only if there was a superior devil within 60 feet of it. That means there’s no longer any particular need for merregons to form a line to either side of a bone devil, erinys, pit fiend or amnizu commander. They can form any kind of formation now, including rank upon rank in front of their commanders, who can lead from the rear. A detachment of them can also break formation to strike at an enemy weakness. Mind you, at CR 4, merregons are hardly weak minions—each of them is roughly the equivalent of a level 11 PC—so even a mere platoon of them is better managed using the mass combat rules of your choice. The Loyal Bodyguard reaction is unchanged, so it does still make sense for a ring of merregons to surround the superior devil that commands them and act as its personal guard.
Half a dozen undead creatures in Volo’s Guide to Monsters and Mordenkainen’s Tome of Foes receive significant updates in Monsters of the Multiverse, and deathlocks account for half of these—unsurprisingly, since they’re all spellcasters.
Gone from the deathlock’s Spellcasting repertoire are arms of Hadar, hold person and chill touch. Eldritch blast is reskinned as the ranged spell attack Grave Bolt, dealing an extra 3 damage (presumably from the deathlock’s Charisma modifier). A new Multiattack lets it attack twice with either Deathly Claw or Grave Bolt, doubling the amount of damage it can deal in a single turn.
These changes turn the deathlock inside out. For starters, it loses both of the spells that benefited from being boosted to a higher level by the deathlock’s warlockitude. It also no longer has anything that fills the role of chill touch’s suppression of healing. On the other hand, the fact that the deathlock now gets to attack a second time makes invisibility-based ambush more practical (although it gains advantage only on the first attack roll of the two), and spider climb no longer has to compete against more potent spells for the use of a spell slot.
As for direct attacks, the choice is no longer between Deathly Claw and chill touch but rather between Deathly Claw and Grave Bolt—which is really a choice between melee and ranged combat. This choice is resolved by looking at the deathlock’s ability scores and asking what they say about its combat role. With Charisma as its primary offensive ability and Dexterity as its primary defensive ability, the deathlock is a spellslinger, and as such, it wants to sling spells and avoid melee.
Therefore, its strategy is now to fortify itself in advance with mage armor and either disguise self or invisibility (the latter precludes the use of detect magic while the deathlock concentrates on it); stay as far as possible from likely foes; cast hunger of Hadar to delay opponents while the deathlock completes its task(s); and if that fails, cast spider climb to escape or to attack with Grave Bolt from inaccessible places. Since the deathlock no longer has a convenient way to paralyze an opponent, Deathly Claw is now only a last-ditch defense, for use when the deathlock is cornered and can’t get out of melee.
Monsters of the Multiverse doesn’t make many changes to giants. Then again, there weren’t many giants in Volo’s Guide to Monsters and Mordenkainen’s Tome of Foes to begin with, just some specially trained and equipped ogres and elite giants and trolls. Only three of these are revised enough to require reexamination.
In contrast to humanoids and NPCs, Monsters of the Multiverse makes few substantive changes to the monstrosities from Volo’s Guide to Monsters and Mordenkainen’s Tome of Foes; most of its changes are too minor to affect the relevant creatures’ tactics. Only the choldrith, leucrotta, yuan-ti and froghemoth receive changes to their traits and/or actions significant enough to merit reexamination.