rocking horse manure
slang Something is extremely rare or unlikely; all but, or most likely, impossible or non-existent. Typically used in the phrase "(as) rare as rocking horse manure." An honest person in politics? Yeah, that's as rare as rocking horse manure. I'm sorry to say, but the part this engine needs is rare as rocking horse manure. They haven't manufactured these in nearly 50 years!
rocking-horse manuresomething extremely rare. Australian informal
See also: manure