"We thought it was a bog-standard chicken until it started breathing fire."
-- Bob (OP7)
Bog-standard is a British slang term meaning “common, ordinary, with no frills”.
The full entry of the Oxford English Dictionary (available via some library systems) contains speculation about the etymology:
Origin uncertain; perhaps an alteration of box standard....
Differing theories of the origin of bog-standard have been proposed, but none proven.... The most commonly held view is that the transition from box to bog resulted from a mishearing or misunderstanding of box standard.Others have suggested a derivation.... bog-wheel, former Cambridge slang for a bicycle, though ultimately also related to bog [meaning "toilet"].
From the Web
Lexico (free Oxford English Dictionary): bog-standard