D&D 5E Do monsters benefit from a long rest like adventurers?

[aramis erak]

Apologies if this is a silly or obvious question but. Well, do they?
My pcs locked an owlbear in a tower as they barely escaped with their lives. After a long rest they went back.
I ruled it did...what say you?

It always does when I'm running D&D...
And note: Since HP are fixed first, and HD derived from the fixed HP total, the HD calculated have only one obvious use: short rests.

Very of the rules are specifically for PC's; very few rules are specifically for NPCs and other monsters. Most of the rules are used by both. (And the DMG makes it explicit that monsters can have class levels added.) NPCs can be built multiple ways, even - either build as if a PC, then find the CR, or build from an existing base NPC by adding a race modifier (in the DMG), or build as a whole new monster entry.)