I know that enemies can take both short rests and long rests, because there is at least one enemy which recover one of its features after a rest. See the Cloaker (Monster Manual, p. 42):
Phantasms (Recharges after a Short or Long Rest). The cloaker magically creates […]
But my question is whether they can spend Hit Dice (if they have them) in order to recover HP.
Take as an example this same creature:
Hit Points 78 (12d10 + 12)
[…] CON 12 (+1) […]
Does that mean they have a pool of 12 dice, each one a d10, as Hit Dice? And when they spend Hit Dice on a short rest, do they add a +1 HP per Hit Die rolled due to its Constitution modifier?
Related: Where are a monster’s hit dice found in the stat block? (asks about hit dice but not if they can use them or add they CON) and What is the “+ 75” in these NPC hit point stats? (asks about the flat number next to the hit dice).
Best Answer
The rules are no different than for PCs. Though the rules for Resting on PHB p. 186 say character, not creature, these are the only rules in the game for how resting works, so all creatures with effects from resting (such as healing) have to use them.
The normal rule is that on a short rest, a character may spend any number of their unspent hit dice to roll them, with their Con modifier applied to each die, to regain that many hit points (up to their normal maximum).
That's how monsters do it too.
So for your cloaker, yes: it can take a short rest to roll any of its unspent 12 hit dice, which are d10s, and for each one its spends to roll, it adds +1 due to its Con modifier.