Pig in a poke. Maybe not the greatest drawing of a pig but I think you can tell it's supposed to be a pig.
Buying something without knowing or finding out it's true value. The expression came about in medieval times when meat was scarce and it was common to carry a small pig in a bag also known as a poke to market to sell. (Side note: we borrowed the word "poke" from the french word "poque" which translates as bag. Interestingly enough adding "ette" to a french word to form the diminutive of the word gives us "poquette" a small bag which we pronounce pocket.)
But back to the story. The less scrupulous of sellers would substitute a cat or a small dog for the pig giving the buyer something worth a lot less than he bargained for. And only when they opened the bag did they find a switch had happened.
Which led to the idiom "letting the cat out of the bag"
