Are they proficient? Is proficiency part of a monster stat block?
— Michael Andai (@MichaelAndai) September 16, 2019
A D&D monster is proficient with any equipment in its stat block. The DM decides whether a creature is proficient with anything else it might wear or wield during play.
For more info, see the "Armor, Weapon, and Tool Proficiencies" section of the Monster Manual (p. 9). #DnD https://t.co/z4AoL0vmKa
— Jeremy Crawford (@JeremyECrawford) September 16, 2019
A DM can also decide that the creature is not proficient with what is in the stat block. Indeed.
— Jeremy Crawford (@JeremyECrawford) September 16, 2019