I've not heard of "jack in the pack." I suppose it alludes to a card deck.
"Dark horse" is an idiomatic expression used to describe someone who unexpectedly achieves success. It comes from horse racing.
John Smith was a true dark horse in the 2000 election, having neither major financial backing, nor overwhelming party support. His come-from-behind win proved that voters were looking for a fresh face and new ideas.
oooooooh, I'm sorry again. you're right it was joker in the pack... something wrong with me today... I guess it's monday and rainy weather
My friend and I were talking about this and that today, and she turned to be this dark horse in one matter. So I said to her, and she replied that she never heard such an expression... But I was sure that there is such an idiom. So I've looked for it in my dictionary, and found it. It means exactly what GenJen54 said. Among it's synonyms was that 'joker in the pack'... hence the question about differences came
ps. There was one more synonym for this - 'obscurity'... yet I never heard somebody using it... what would it mean in regards to a person?
Dark horse : 1. a person who does not tell other people about their ideas or skills and who surprises people by doing something that they do not expect. I didn't know Linda had written a novel. She's a bit of a dark horse, isn't she? 2. a person who wins a race or competition although no one expected them to. 17-year-old Karen Pickering could also be a dark horse for (= she could win) a medal in the European Championships. (sometimes + for) http://idioms.thefreedictionary.com/dark+horse
I think these definitions help clarify it. A dark horse has an affect on his or her own results. A joke in the pack could affect the result for others and does not always affect themselves.