Hm. Well, if that's the case, then you could probably just use the MuCo(An) - "grant aspect of an animal" guideline (eyes of the cat, for example) to give yourself "stamina of the boar".
Although from what I recall of the implementation, the guideline actually grafts or replaces an animal body part onto the human body: the specific effect (seeing in the dark with cat's eyes, for example) is just a convenient side effect. So how much of the physical aspect of the boar you'd get is an issue - would it be just the torso? Would you go all bristly and grow tusks?
Although 'stamina' may be represented by the heart(?) - so "heart of the boar" may be enough.
From what I recall of the text, "minor" refers to "how much of your body physically changes", not "how useful it is." The utility of the change is dependent on the circumstance and the individual being changed, not the level of the effect. For example, "eyes of the cat" is crazy-insane powerful....if you're in the dark and being chased by ten other equally night-blind bandits.
That being said: if gaining an additional level of boar stamina requires a Bebop or Rocksteady-style of transformation (https://en.wikipedia.org/wiki/Bebop_and_Rocksteady), then yes: that would probably go under the "full body transform" category. But if it can be shown that (say) only gaining the heart or lungs of the boar would suffice (based on a medieval understanding of stamina), then it should be covered.
There is no evidence of any such language in the text that I can see.
And while I agree that the ability to see in the dark is a pretty big deal, it also presumes that you have cast it on everyone, otherwise the rest of your party is going to have difficulty navigating like the other guys. I have something of a problem with a MuCo(An) spell of 5th level granting an extra level or two of fatigue. That's not minor, given how useful fatigue is.
I'm basing it on later guildelines: (Serf's Parma) - later Muto effects are "full changes" or "more complex changes" (ie, "transform into wolf"), not "more useful changes".
Also - useful for your playstyle, perhaps. Personally, I think I've spent fatigue only a handful of times, and no more than 2 in any given scene. (Mainly I play lab rats and hyper-specialists who don't need to spend fatigue to succeed in their rolls.) As such, an extra level of fatigue has never been particularly useful for any of my characters. So, for me: yes, it would be minor.
As such, (taking it to an extreme), equating "minor" to "useful" would require that the power have an implied intelligo effect, in order to determine how useful the effect is in-game. Or alternately, it would require some sort of meta-ability to identify plot points and theme and how useful it is in the given scene. Now, there might be some sort of Mythic Europe objective definition of "useful" here, but I'm not aware of one offhand. As such, it seems more reasonable to equate "minor" with the size of the transformation, rather than a more esoteric "utility to a task I might want to perform at some point."
EDIT - but again: if you think that an increase in stamina would require a full-body (or near-full body) transformation, then yes - it would be higher. I just don't necessarily think it would be. (Another example: you can one-shot a magus with a lvl 5 CrTe "create temporary rock over their head" effect, if you've got enough penetration. Personally, I wouldn't normally think that a mag1 effect could kill someone, but that's how the system works. Therefore, equating level with utility seems a bit suspect, to me.)
If all you play is lab rats, then an extra fatigue level is nearly meaningless. If, instead, you are playing a combat maga who can life boost her spells to get to impressive penetration levels, then extra fatigue is pretty nice. For physical combatants, it's great to boost your attack or defense totals. Adding another level, means you do it longer before you have to worry about fatigue penalties. It's a low enough level spell that one would cast it on grogs all day, everyday in the field. It's even better than a modified Gift of the Bear's Fortitude.
I don't agree that the utility of the spell necessitates an intellego requisite, free or otherwise. I actually find that a bit of a red herring, how does one know changing into a wolf is more advantageous in any given situation? But adding a fatigue level or two to any individual for D:Sun is nearly a game changer if there are any combat situations that are involved, and certainly by magi who expect to cast a lot of spontaneous magic...
Yes. Which is why "how useful" something is isn't a good measure of how to rate the difficulty of a spell. Utility is context-based: what is useful for one magi isn't useful for another. As such, deciding that "a spell is too useful - therefore, it must, by definition, be more difficult" is a bad rating. Or would you argue that because it's not useful to a lab rat, it is therefore easier for that character to cast? Because that goes against what I understand of how the game mechanics operate.
Yes. That's my point. Which is why I interpret "minor" to mean "small physical change, regardless of how useful it is" - it's more consistent with how the rest of the guidelines are described. And as you say - it avoids the whole "how does one know if changing into a wolf is more advantageous?" You don't need to. Utility is in the eye of the beholder, and as such is a bad reference for an objective guideline. In contrast, "size of the physical change" is more objective, and is the same for everyone, and matches up more consistently with the other descriptions of the TeFo. Therefore, "minor" probably means "minor physical change" rather than "only mildly useful."
Minor physical change is subjective, too. Or at least not implemented consistently.
Eyes of the cat changes the eyes, allows one to see in darkness. The spells in Magi of Hermes change legs, and other body parts, and as I recall provide a numerical bonus. Why is changing someone's legs minor, or on the same level as changing someone's eyes? How is that an objective standard?
OK, just looked up the guidelines in AM5, and the examples in MoH - seems pretty consistent to me:
Muto Corpus
2 - Grant someone a minor ability
2 - Grant someone a minor ability from an animal (Animal Requisite) <- never directly stated that I was able to find, but it seems to be implied.
10 - turn someone into a land animal (Animal requisite)
I wasn't able to find an example of the leg one that you described in MoH - at best, there was a straight-up MuCo one that gave the ability to "run as fast as a horse" - but it didn't actually grant the legs of a horse. Of course, it didn't have an Animal Requisite either. So that looks like it was just "granting a minor ability" via the straight-up Corpus rules. In looking at the others: (pg. 85, mainly) - feet, hands, outer skin, and tail. The one exception is the "gift of the Minotaur" (Pg. 85), which turns someone's head into a Bull's. That one seems to use the Base 10 ability, with target: Part.
So there's your answer: a human head requires the base 10; anything smaller seems to use the base 2.
With that in mind, it looks like the basic "grant minor ability" means "grant single ability that is not very far away from the standard ability of the human body". In contrast, the version with the animal requisite is "grant single ability that is an innate power of the animal part that gets grafted onto the body".
EDIT - but in thinking about it more, I will agree that a +1 fatigue level is more likely to be a full-body effect, even in the medieval paradigm - it requires not only the heart and lungs of a Boar (for example), but also various muscles. So I'd say it would be based on the lvl 10 guideline, although there probably wouldn't be any OBVIOUS changes - the effect isn't asking for tusks or bristly hair or anything - just the stamina.
I still stand by my statement that granting extra fatigue is more than a minor ability, because of how useful fatigue is for mundanes and magi alike. That's not intellego, that's my opinion as an SG, and the opinion I would submit to any troupe I'm part of.
And my argument is that the definition of "minor" is more consistent when rationalized with the "complete" of later guidelines, rather than "useful".
But, as I edited above, I do agree that a +1 fatigue level is more likely to be a lvl 15 effect (base 10 Total Transformation, +1 part), simply because it affects the entire body.
EDIT - the MuAn guidelines seem to bear this out:
Base 3 - change an animal's limb
Base 4 - make a major change to an animal, while leaving it substantively the same animal
Base 5 - make a minor unnatural change (full-body, I think)
Base 15 - make major unnatural changes (again, full-body is implied)
Base 25 - make radical unnatural changes. (full body again implied)
So if we allow ourselves to use the MuAn as a parallel guideline for MuCo(An), the base 3 is required to affect an entire limb, while base 2 allows you to affect anything smaller (hands, eyes, feet, ears, nose, and I suppose the outer dermal layer.)
So the guidelines seem to be based not on how useful the change is, but how much of a change they are - ie, how far away they get from the base you start with. And the MuCo(An) base 2 effect seems to let you start with the ability that the animal part in question grants.
What is the actual limit? As Jonathan.Link said, I thought it just prevented faster regeneration - not from having a deeper well to start with. For example - would you allow a CrCo to heal an injury that had taken a permanent fatigue level from someone? Personally, I would (Serf's Parma - unless the actual description explicitly prevents this) - and while that starts to get into "Creo only allows you to attain the Platonic Ideal of a target," the point is that it allows someone to increase their fatigue reserves.
My personal interpretation (assuming it's not directly prohibited) is that the Limit of Energy is a lesser Limit - by canon, it was something Bonisagus had developed, but got stolen from him by that Amazon gal. So it's not like it's impossible to do. (Along a similar vein, it's supposedly impossible to detect a Demon - except you can do that just fine by shooting your suspect with a Demon's Eternal Oblivion, and observing if the target starts to melt.) Thus, "limit" seems to not actually mean "limit" - rather, it usually seems to mean "not directly possible using the way you might think it should work."
Considering that art and academe allows tonics that restore fatigue, and those can be mixed with craft magic...
but that isn't the same as actually getting an extra level of fatigue.
Well, it can potentially apply - if your GM rules that your character can quaff a potion in the middle of a ritual, I suppose. And yeah, it looks like you can create the Tonic of Gold (pg. 77), which can restore the 1st level of fatigue (as long as you haven't lost any more fatigue other than that.) Although it does require a finesse roll of 16 to pull off the Craft Magic; probably could make it a bit easier if you slowed down a bit. More powerful tonics affect other levels of fatigue, of course. So the theoretical artist who is creating his great work can (in theory) have a couple tonics sitting off to the side - the mideval equivalent of Red Bull, and slam them down every so often. They do seem a bit pricey, but I'd imagine that a magus set up to sell these off can make a tidy profit...