Don't know how to quote, but LOL at "one's horse"!
Well, no-one sat down with a big book and invented English grammar and decreed this is how it is and how it will be forever more. English is just a mish mash of latin and french and anglo-saxon, the way it came to be spoken over hundreds of years. Printed media cause it to be frozen in time to some extent, but in the end if everyone (and I mean newspapers, style guides etc) come to use a particular construction in a particular way that IS the new correct. Never seen "would of" in print, so don't think that's going into the text books any day soon :)
There are many example of false usages of words which were originally a mistake ("nice", for example, used to mean scrupulous, precise or exact), but because common usage changed the new meanings are in the dictionary. Same happens with grammar.
Interestingly the German authorities decreed some changes to spelling a few years ago. They're struggling to get it to stick from what I've seen, and again I may be a few years out of date, because common usage is against them. The French authorities periodically try to prevent too many American/English borrow words from coming into the language, but they can't stop them. Common usage.
I wouldn't actually swear to "an hill" being a universal old usage, but that's what was taught to us at primary school, and I've seen enough corroboration to believe that at least used to be taught as correct and wasn't just one weird teacher. I never said it didn't sound silly or that I would use it 