Snopes.com reports that when offered a pig open the poke (c. 1325) points to the origin of this phrase. I have not found the source of this in Google searching. It may be in the OED or some other reputable source. As the date puts the expression in Middle English, it is highly likely that the the above proverb is a paraphrase or has some modernized spellings. DCDuring TALK 15:28, 18 December 2010 (UTC)
Origin: makers of fur coats?Edit
The same expression exists in Dutch ("een kat in een zak kopen" = to buy a cat in a bag). The French version of this expression also mentions a cat. This page in Dutch discusses two possible origins:
It says that fur dealers used to buy cats to use their fur, but only black cats were suitable. A careless dealer might get tricked buying a cat in a bag without checking that it's indeed a black cat.
The page then gives a second possibility, that it comes from an old story about a never-ending coin that you could get by selling a cat in a bag to the devil, telling him it's a hare. Gronky (talk) 06:06, 22 November 2013 (UTC)