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.
The feat always benefits smaller hit dice more, but by how much depends on your Constitution modifier. The average increase is 0.60454545 Hit Points.
I'll first present the math/formulas, then the raw numerical results and then the more natural English generalizations with a graph. (you can scroll down to go straight to the results).
The section on "Short Rest" states:
For each Hit Die spent in this way, the player rolls the die and adds the character's Constitution modifier to it. The character regains hit points equal to the total (minimum of 0).
And the Durable Feat states:
When you roll a Hit Die to regain hit points, the minimum number of hit points you regain from the roll equals twice your Constitution modifier (minimum of 2).
This increases our minimum to the higher of double our Constitution modifier (unless our modifier is zero or less in which case the minimum is 2).
Because of the negative modifiers there are actually two separate formulas we will have to use:
- First, let our Constitution modifier be between -5 and 0.
If we had a d6 Hit Die and a -3 Constitution modifier our possible Hit Point gains would be:
0, 0, 0, 1, 2, 3.
There will always be a number of zeroes equal to the negative of our Constitution modifier. Here we have zeroes until we roll a 4 on the die as 4-3 (the die result plus our Con modifier) is 1.
Using the Durable Feat our new possible Hit Point gains would be:
2, 2, 2, 2, 2, 3.
The new minimum is two, so we have increased every result that was less than 2.
In particular, all of the zeroes are replaced by twos and the single result of one that we had is also replaced by a two.
Thus we have increased the total by 2 times our negative Constitution modifier plus 1: -2MOD+1.
Dividing this by the number of results (as they are all equally likely) will get us our average increase from the feat, (HD represent the size of our Hit Die):
(-2MOD+1)/HD.
- Now, let our Constitution modifier be between +1 and +5.
If we had a d6 Hit Die and a +3 Constitution modifier our possible Hit Point gains would be:
4, 5, 6, 7, 8, 9.
The lowest we can roll will always be 1 plus our Constitution modifier.
Using the Durable Feat our new possible Hit Point gains would be:
6, 6, 6, 7, 8, 9.
The new minimum is double our Constitution modifier, so we have increased every result that was less than that number.
The first result will increase by our Constitution modifier minus 1 (double our con mod minus the sum of our con mod and 1), the second result wil increase by our Constitution modifier minus 2, and so forth.
So will get every number from 0 to out constitution modifier minus 1. Summing these numbers we get this: (MOD-1)(MOD)/2.
Dividing this by the number of results (as they are all equally likely) will get us our average increase from the feat, (HD represent the size of our Hit Die):
(MOD-1)(MOD)/(2HD).
The raw numerical results
Your average increases by (-2MOD+1)/HD for modifiers less than +1.
Your average increases by (MOD-1)(MOD)/(2HD) for modifiers +1 and above.
I had a python script run these formulas over the range of hit die sizes (from d6 to d12) and constitution modifier values (from -5 to +5) and got the following results:
You always gain a greater amount of Hit Points from the roll if your Hit Die size is smaller.
The x-axis if your Hit Die size. The y-axis is your Constitution modifier. The values are the gain that you get from this feat.
| Modifier |
d6 |
d8 |
d10 |
d12 |
Average |
| -5 |
1.8333 |
1.375 |
1.1 |
0.91666 |
1.30625 |
| -4 |
1.5000 |
1.125 |
0.9 |
0.75000 |
1.06875 |
| -3 |
1.1666 |
0.875 |
0.7 |
0.58333 |
0.83125 |
| -2 |
0.8333 |
0.625 |
0.5 |
0.41666 |
0.59375 |
| -1 |
0.5000 |
0.375 |
0.3 |
0.25000 |
0.35625 |
| +0 |
0.1666 |
0.125 |
0.1 |
0.08333 |
0.11875 |
| +1 |
0.0000 |
0.000 |
0.0 |
0.00000 |
0.00000 |
| +2 |
0.1666 |
0.125 |
0.1 |
0.08333 |
0.11875 |
| +3 |
0.5000 |
0.375 |
0.3 |
0.25000 |
0.35625 |
| +4 |
1.0000 |
0.750 |
0.6 |
0.50000 |
0.71250 |
| +5 |
1.6660 |
1.250 |
1.0 |
0.83330 |
1.18750 |
| Average |
0.848484 |
0.636363 |
0.5090909 |
0.424242 |
0.60454545 |
There is also the notable case of a level 20 Barbarian as their Constitution modifier can reach +7 on their d12 Hit Die. For them the feat increases the average by 1.75 Hit Points.
The generalizations
The feat benefits the a negative modifier more than its positive counterpart, but this is because the feat provides no increase for any dice if you have a +1 modifier:
If two modifiers are the same distance from +1 (like +3 and -1), the positive modifier will benefit from the feat equally, unless the modifier is +4 or above, in which case it will benefit from the feat more than the negative one.
The graphical representation
The x-axis is your modifier, the y-axis is the gain from the feat. The smaller hit dice have the higher benefits.
Purple is the d6, Green is the d8, Blue is the d10, and Red is the d12.
Best Answer
A sidekick starts with as many Hit Dice as noted in its stat block.
The rules about applying a sidekick class to an NPC do not state anything about removing Hit Dice from a creature before applying the class, nor does it specify the starting Hit Points as every player class shows. Rather, a sidekick starts with the Hit Die/Dice appearing in the stat block.
Quoting the relevant parts from Tasha's Cauldron of Everything (page 142), in the paragraph Creating a Sidekick:
As an example, a 1st-level sidekick wolf would start with 2d8 Hit Dice. Similarly, a 1st-level black bear would start with 3d8 Hit Dice. In each case, those two sidekicks would gain an extra Hit Die (a d8) for each new sidekick class level.
The Monster Manual specifies that those dice are, in fact, Hit Dice at page 7, in the paragraph Hit Points:
Sidekicks classes are not player classes. Each player class specifies the starting Hit Die and Hit Points at first level, but the sidekick classes do neither. You just take the stat block and "add to it".