I'm going to use one example just to demonstrate the relevancy of this question, but I want to make clear that this one example does not constitute the complete domain of my question; nor do the other questions I linked in this post constitute the whole picture of what I'm asking, but instead are just illustrative examples.
The Inciting Example
For a creature like a Panther, their Dexterity Bonus is +2, and their CR is 1/4, meaning they'd normally have a Proficiency Bonus of +2, meaning Dexterity-based skills that they have Proficiency in should have a bonus of +4. However, for Stealth, they have a total bonus of +6.
Mechanically, this is justified: the Monster Manual expressly calls out monster skills as potentially subject to greater-than-expected bonuses, borne out of expertise in the given skill:
A skill bonus is the sum of a monster's relevant ability modifier and its proficiency bonus, which is determined by the monster's challenge rating (as shown in the Proficiency Bonus by Challenge Rating table). Other modifiers might apply. For instance, a monster might have a larger-than-expected bonus (usually double its proficiency bonus) to account for its heightened expertise.
—Skills, Monster Manual, pg. 8
So we have a clear explanation for why a Panther would have an unusually high Dexterity (Stealth) bonus of +6, but this also leads to some strange territory in terms of game mechanics. For example, if a Panther is tamed by a Beast Conclave Ranger (as seen in Unearthed Arcana: Revised Ranger), the Panther would substitute their master's Proficiency Bonus for their own for their skills. This leads to an ambiguous situation regarding what bonus the Panther should have to this skill:
- 4, because you use the Ranger's Proficiency bonus, replacing the Panther's getting 2(DEX) + 2(PROF)
- 6, because you use the Ranger's Proficiency bonus, and then double it, because the Panther has expertise in Stealth, getting 2(DEX) + 2x2(PROF)
And then at level 5, this situation arises again, does the Panther have
- 5, because you use the Ranger's Proficiency bonus, replacing the Panther's getting 2(DEX) + 3(PROF)
- 8, because you use the Ranger's Proficiency bonus, and then double it, because the Panther has expertise in Stealth, getting 2(DEX) + 2x3(PROF)
And so on at higher levels, does this ambiguity continue to arise.
This is a relevant question for lots of other classes as well. Druids, for example, are able to Wild-Shape into various beasts; a Druid could shape-shift into a Panther, and then the Druid's Proficiency Bonus (which at most levels of play will be higher than the Panther's proficiency) might or might not get doubled: see this question for a concrete example: When a druid wild shapes to a beast that has double proficiency with a skill, should the druid's proficiency bonus be doubled with that skill?
'Expertise' vs 'expertise'
In a recent answer I gave about Aboleths having a doubled proficiency bonus in History, I very explicitly avoided saying that Aboleths "have expertise" in those given skills, because I was worried that the answer might get downvotes over arguments about the semantics of a creature "having expertise" or not, and instead I focused on describing them as "having something that is mechanically equivalent to expertise" in those skills.
So what I'm most curious about is the use of that original passage I quoted above. It connects a creature potentially doubling its Proficiency bonus to the creature having 'expertise' in a given skill, but it's not made clear that 'expertise' represents a tangible feature of the creature in question. Ordinarily, I would be inclined to argue that the term is meant to just be a colloquialism, but then we know that Bards and Rogues have an explicitly named feature, Expertise, that explicitly allows these classes to double their proficiency bonus on specific checks.
Then there's a feat featured in Xanathar's Guide to Everything, Prodigy, which strongly implies that 'expertise' is a concept, not merely a colloquialism:
Choose one skill in which you have proficiency. You gain expertise with that skill, which means your proficiency bonus is doubled for any ability check you make with it. The skill you choose must be one that isn't already benefiting from a feature, such as Expertise, that doubles your proficiency bonus.
—Skilled, Xanathar's Guide to Everything, pg. 75
So it seems very strongly like the word 'expertise' has a Formal Ontology in 5th Edition D&D, meaning "double your proficiency bonus for checks made using this skill", and is not merely a colloquialism for "your bonus in this skill is higher than expected".
But then I look at answers, like in the Druid question I linked, that seem to imply that this is not the case; that it really is a colloquialism, and that there is no formal concept of a feature that doubles a creature's proficiency bonus, except in the very specific case of Rogues and Bards with their explicitly named feature, Expertise, providing such a benefit. And of course, my Aboleth answer above focused on the same kind of rhetoric, without receiving pushback on the semantics.
So which is true? Does the word 'expertise' have a formal ontology in 5th Edition, or doesn't it?