So it says followers of erastil can summon a celestial dire boar as a summon monster 3 choice.
My question is can you use versatile summon monster feat to summon a chthonic dire boar as a choice at that lv?
It stats
"When you summon one or more creatures that would normally be available with the celestial, entropic, fiendish, or resolute template using a summon monster spell (or an effect that mimics such a spell), you can instead apply one of the chosen templates to each creature. You can apply a different template to each creature you summon"
Would that wording allow it?
I am not sure it is legal to do so as the entry specifically says « Celestial Dire Boar » rather than « Dire Boar* ».
However, I do not understand why the writer specified the Celestial template, as clerics of Old Deadeye can make use of the Resolute template as well. Maybe Kingmaker was published before the Monster Summoner's Handbook, a time when the Resolute template wasn't yet available to clerics of Lawful deities.
If this is the case, the entry for summons peculiar to Erastil should be revised to reflect the choice in template, in which case, Versatile Summon Monster works.
If the M.S.H. was out already then the writer decided that clerics of Erastil using S.M.III to summon a Dire Boar could only get a Celestial one, while S.M.IV could get them either a Celestial or a Resolute one, in which case V.S.M. doesn't apply.
In this case, I would rule that it works nonetheless - and that choosing the Resolute template is possible.