"Pony up" means basically to pay up. It is generally used in a group context, but is appropriate in a one on one arrangement as well. It's a slang term, generally used in a situation such as this:

Joe is elected to go on a food run. He goes out to Krispy Kreme donuts and picks up two dozen donuts, which costs him $12.00. He brings the donuts back to Company X and lets his 11 fellow employees know how much he spent on donuts. 10 employees give him a dollar, and the 11th pretends not to have heard him as he finishes off his third powdered sugar donut. Joe can feel justified walking up to No. 11 and telling him, "Pony up, fatass."

Not that Joe would ever call anyone a fatass, but the example works well. This term could have come from card game lingo "ante up," but I have not seen any research to prove or disprove my theory.