I'm comparing Giant Badger (MM, p. 323) to Panther (MM, p. 333), with my druid's Wild Shape in mind.
A Giant Badger has Multiattack, Bite and Claws. A Panther has Bite and Claw.
Since Multiattack does not appear for the Panther, is it correct to say that it must select Bite or Claws as its action, but not both?
Monsters possible actions are described in more details on page 10 and 11 of the Monster Manual.
Multiattack
A creature that can make multiple attacks on its turn has the Multiattack ability. [...]
So clearly, if the description includes a Multiattack section, then multiple attacks can be performed. You have to be careful as it is not always by different weapons. That is, the multiattack may be twice with the same weapon. Some monsters can also use more powerful attacks only once (such as breathing fire) whereas, it may be able to do a multiattack once with a claw and once with a bite.
Correct, multiattack tells you which attacks you can use simultaneously, whereas a creature without it is limited to choosing between the attacks it has available.
There's actually nothing more to add as you covered it appropriately within the question itself.
