In general you just don't want to overload anything that's just intended for a one-off fight with features since they'll usually only get 2-4 turns to act- for better or worse, this was a large part of the logic in trimming out NPCs with spell slots that prep/learn spells in about the same way PCs do in the new MM. If this is a recurring figure, adding on extra capabilities to fit a story beat or just make them distinctive individuals is pretty common.
I've been thinking that a red Dragon can have the Boon of Terror, can that happen or no?
If you're the DM, the monsters can have whatever features you want.
pronouns: he/she/they
In general you just don't want to overload anything that's just intended for a one-off fight with features since they'll usually only get 2-4 turns to act- for better or worse, this was a large part of the logic in trimming out NPCs with spell slots that prep/learn spells in about the same way PCs do in the new MM. If this is a recurring figure, adding on extra capabilities to fit a story beat or just make them distinctive individuals is pretty common.
They absolutely can. The members of the Hourglass Coven (Skabatha Nightshade, Bavlorna Blightstraw, and Endelyn Moongrave) and Iggwilv the Witch Queen all possess the Boon of Immortality. They're from The Wild Beyond the Witchlight.
They/Them
My Homebrew