Are there any monsters that can target more than one target/creature?

Question

I noticed that the attacks in Statblocks mention one target (and sometimes one creature). This made me curious. I have been poring over the MM and VGtM, but have not found any monster that can make a melee/ranged attack against more than one target / one creature.

1. Do we know of any example within the 'official' books where this is more than one?
2. If it's always one, do we know of any reason why this is at all mentioned in the statblocks?

Related: Are there any issues with creating creatures that can make multi-target melee attacks?

Related: In the descriptions of monster action options, what's the difference between "one target" and "one creature"?

Answer 1

There was one: Deathlock Mastermind (MToF, pg. 129).

The Deathlock Mastermind's attack Grave Bolts says:

Grave Bolts. Ranged Spell Attack: +6 to hit, range 120 ft., one or two targets. Hit: 18 (4d8) necrotic damage.

I have searched through all three primary monster source books and this is the only one I found.

Monsters of the Multiverse changed "Grave Bolts" to "Grave Bolt".

With the release of Monsters of the Multiverse, Modenkainen's Tome of Foes is obsolete. The Deathlock Mastermind received an update that changes how Grave Bolts works. Instead of Grave Bolts having "one or two targets", it now targets one creature but can be used twice:

Multiattack. The deathlock makes two Deathly Claw or Grave Bolt attacks.

Grave Bolt. Ranged Spell Attack: +6 to hit, range 120 ft., one target. Hit: 13 (3d8) necrotic damage.

Answer 2

Future-proofing (up until MToF), and just being explicit for clarity

Thomas Markov's answer shows that there is one monster in Mordenkainen's Tome of Foes that can target more than one creature; the Deathlock Mastermind. However, before that, the only reason for explicitly stating what is always true for each attack in Monster Manual and Volo's Guide to Monsters must have been for future-proofing, in case one day they bring out a new monster, like they eventually did in MToF.

It's also perhaps simply good practice for them to be explicit to remove any doubt that an attack was intended to target only one creature (an easy mistake for players or DMs to make when considering gargantuan creatures; I can easily imagine someone thinking "but this creature is massive, of course its tentacle attack could hit all of the party members at once" were it not explicitly stated).