What happened

Shares of Chinese electric-vehicle maker NIO (NYSE:NIO) had a rough ride last month. According to data provided by S&P Global Market Intelligence, the price of NIO's U.S.-traded stock fell 17.4% in December.

Part of the drop was almost certainly due to the larger marketwide sell-off we saw last month, when the benchmark S&P 500 dropped a bit more than 9%. But NIO's shares took a much bigger hit due to growing concerns about the health of China's new-car market -- and the possibility that Tesla (NASDAQ:TSLA) could be more competitive in the Middle Kingdom than previously expected.