Pigs, it seems, get a very bad rap, as witnessed by pig idioms.
fat as a pig
exceptionally fat; grotesquely fat.
Rur. is or are filthy or uncouth. After a day's work in the hot sun, Clyde ain't fittin' to roll with a pig.buy a pig in a poke
Fig. to buy something without looking inside first. If you don't get a good look at the engine of a used car before you buy it, you'll wind up buying a pig in a poke.
Rur. That's amazing!
In a pig's ear!
Rur. very easy.
Fig. to eat too much, too fast, or noisily; to eat more than one's share.
Inf. to eat too much of something; to make a pig of oneself.
When will this defamation of pigs cease?
When pig fly.
Pig idioms courtesy of The Free Dictionary.
Flying pigs courtesy of Cornerstoregoddess here and here.