"to buy a pig in a poke"

This is a phrase which dates back to the Late Middle Ages of Britain. At this time, meat was scarce and confidence tricksters would sell ‘pigs’ in unopened pokes (bags) to unsuspecting consumers. When the buyer returned home, they would find a cat or a dog in the bag instead of the promised pig. The phrase passed into everyday language, referring to buying something which turns out to be quite different to what was promised.