Sometimes we have to scrape the bottom of the barrel to figure out which form to use.  I don't think there is any statute, regulation, revenue ruling or rev proc that addresses the issue of when to use C or F.  All I can find is something in the Schedule F instructions that, as you point out, mentions pet boarding goes on Schedule C.  Are horses pets?  Does IRS even acknowledge the existence of horses?  There is no Principal Agricultural Activity Code for them.  You have to throw them in with alpacas and minks.