Why pigs?
'Cause we’re glad that cows don’t fly.
If you didn’t care . . . .
Bacon is going up.
Pigs Fly and Cows Jump (over the Moon).
Probably because they are way, way too fat to fly.
You could probably put wings on a shorn sheep and it would glide for a bit if you tossed it off a cliff, and there is Pegasus, but a big old fat pig?
No way.
It occurs to me that a pig shares the aerodynamic characteristics of a wingless bumblebee. If one could somehow crossbreed pigs with bumblebees, thus producing a winged pig, perhaps the result would be self-delivering bacon…
With honey…but there is a mass-size ratio problem here. I mean, pigs are pretty dense, too.
You know that I care…
Pigasus
Hm. Should thesebe considered pigs that fly?
Of course.
That just doesn’t go to the etymological question you posed.
I’m working on it…
because we already have horseflys, houseflys and dragonflys.
Ever try to book a pig a first-class ticket to London?
Flyin’ into London from over the Pole,
Flyin’ in a big airliner…
-Arlo Guthrie.
“This is what it sounds like
When pigs fly”
Just doesn’t sound right!
Harold 
Could you be more specific?
Pigs will fly when hell freezes over.
Of course, this hasn’t quite been tested since Jesus cast out Legion 2000 years ago.