Flank =/= Hindquarters (A Light-hearted Rant) · 6:32pm Nov 16th, 2016
At the risk of pedantry, y'all need to get yourselves a'googling for Actual Horse Anatomy. 
I mean, the show gets it wrong too. Nobody knows how to read a wikipedia page, even. 
It's just constantly jarring having to remember 'oh yeah, they don't mean the area between the ribs and the stifle' when reading fic here. *wags finger with disapproval* Flanks are horse waists, not rumps!
It's probably not a problem that most people have but for anyone who owns real horses (I envy and fear for you in equal measure), or ever had an Intense Horse phase as a kid (me) and consumed all sorts of horse-related media with an obsessive fervour, it's really weird. I can't tell you how many hours I spent trying to remember the difference between cannon and pastern, what the chestnut was for, whether the tail was the croup or the dock (it's the dock), and all of that. Horses have knees on their front legs (on humans, these would be the wrists), and stifles on their hindlegs (which are the knees)!
But I guess blank-flank is catchier than blank-hindquarters, huh? 
This is one of the most fantastic rants I've ever seen on this site. If the show gets it wrong, I'll stick with that for consistency, but it's a great rant all the same!
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Ahaha, thanks! I try, I try.
Yeah, it's easier to stick with what we've got in-show. And that's fine! Pony language seems to be a little bit different from English - the use of 'somepony', 'anypony' etc are a big indicator of such. So you can easily chalk it up to a quirk of alternative language evolution.