Unfortunately, none of them is the idiom I know.

However, I'd like to guess a litte, although I think most of guess about an idiom will be just a guess.

"when pigs fly"

Since I don't think pigs are able to fly, perhaps it indicates that something is impossible to happen.

Or if I think about it in a positive way, then it may be related to a miracle or something like that?

"

"bite the dust"

This is an interesting one. I believe in my mother tongue there is a similar expression. But I'm not sure whether or not they have same explaination.

Literally, if you make others bite the dust in a race, then you run much faster than them and they have to bite or eat your dust.

Otherwise, not limited to race, in any situation when you are competing with someone(s),  this idiom means you are so outstanding that you leave all others behind a lot.

Anyway, I hope this interesting topic to go along. ;)

Never say never.