Multiple monster have a variant of the "Change Shape" ability.
For example, the Night Hag can transform into a small or medium sized female humanoid:
Change Shape. The hag magically polymorphs into a Small or Medium female humanoid, or back into her true form. Her statistics are the same in each form.
Similarly, the Deva can change into a humanoid or beast:
Change Shape. The deva magically polymorphs into a humanoid or beast that has a challenge rating equal to or less than its own, or back into its true form.
Can Change Shape be used to impersonate a specific NPC?