A Question About Monsters and Class Weapon Proficiencies.

[3WhiteFox3]

Ok, so as a DM, I have occasionally given monsters class levels in order to beef them up a bit against high-level characters. In doing so, I also gave them the weapon proficiencies of the class. However, after looking at the RAW, I found that only humanoids gain proficiency with weapons according to character class. Is there a rule somewhere that I'm overlooking?

[Urpriest]

The entry in the humanoid description merely reminds you that, since most humanoids in the monster manual have character classes, their proficiencies are those of their class. It also means that, while a humanoid with no class levels is proficient in simple weapons, a humanoid with class levels is only proficient in those weapons that their class gives them proficiency with, so an Elf Wizard, for example, is not automatically proficient with all simple weapons.

For non-humanoid monsters, that particular clause is absent, so they keep their racial proficiencies while also gaining the proficiencies given by the class.