This is a terribly munchkin question - which mundane animal(s) have the most fatigue levels (ties broken in favour of those with the lesser penalties)?
What I tend to see on the forum is the boar (post-errata): OK, 0/0/0/0, -2/-2, -4, Unconscious
(thus, 7 levels between OK and Unconscious; other animals have 7, but with greater penalties).
On the other hand, the excellent Animals of Mythic Europe by Itzak Even and callen (you should make it more visible, by the way) shows the (domesticated) donkey doing better, with 8 levels between OK and Unconscious: OK, 0/0, -1/-1, -3/-3, -5/-5, Unconscious
However, I guess the donkey is fan-made ... unless it's derived from some other animal with "official" modifications?
The sarda whale gets an extra 4, though it's big so a MuCo(An) spell would need extra size, and a Bjornaer would need Giant Blood and permission. The grey seal gets an extra 3. I think the running dog also gets an extra 3.
So the winner is (unsurprisingly) the whale, followed by the boar (that gets, if I am not mistaken, the same number of extra levels of seal and running dog, but with lesser penalties). Hmmm, a giant-blooded, whale Bjornaer, specializing in Aquam... that's an interesting concept.
Of course, if this is for a Bjornaer rather than MuCo(An), Skinchanger, Shapeshifter, or Lycanthrope, you get access to all the extra Fatigue levels if you get to Inner Heartbeast. If you're headed that route, then take whatever suits you best, don't worry much about the Fatigue levels, and just add the missing ones later.
Separately, I'm still a big fan of the wolf for it's combination of good Fatigue and a bunch of other things.
On the next update I'll see about giving Callen a greater credit for his amazing work.
And I'll have to review the fatigue levels of all animals. I think the ones on the Boar are incorrect, if that's the official stats. So I might go over all animals, and put notes on that where it's needed.
Your stats for the Boar matches what is in the published RoP:M, but there is errata for the fatigue of the boar which had been calculated incorrectly.
The correct fatigue levels for the boar are as listed in the top post here:
Ok, 0/0, 0/0, -2/-2, -4, Unconscious
(Three extra fatigue levels from qualities, and reduced penalty from Enduring Constitution)