Green’s Dictionary of Slang

(orig. Aus.) something extremely hard, if not impossible to find.

1937		Dampier Herald 12 Aug. 2/2: [T]he growth of motor transport in the city had made stable manure almost as rare as the fabulous rocking horse manure.
1967		‘Whisper All Aussie Dict.’ in Kings Cross Whisper (Sydney) xxxix 4/4: rocking horse manure: A very scarce commodity.
1983		J. Byrell (con. 1959) Up the Cross 105: To Klaus [...] Brutus [i.e. a dog] was worth his weight in rocking horse poop.
2001		S. Hughes speech at UK Labour Party Conf. 2 Oct. 🌐 Indeed, you’d have more chance of spreading rocking horse manure on your roses than you would of seeing Michael Howard as President of the European People’s Party.
2016		N. Cummins Adventures of the Honey Badger [ebook] VITAL AUSSIE VERNACULAR Rare: Scarce as rocking horse shit.