'Pig in a Poke'

Meaning that someone has been sold something that is not exactly what they thought they were getting - the derivation is said to be from the Middle Ages when unscrupulous market sellers asked for a suckling pig would hand over either a butchered cat or dog in the bag instead of the more desirable and expensive pork product - it is also a situation that has generated the other related phrase of being 'sold a pup' - (sorry no full blown pigs in the house to be photographed so the Guinea Pig versions were the only option )