A bad year for one veteran of the $3.6 trillion market for exchange-traded funds is only getting worse. Elkhorn Capital Group, which once oversaw more than $200 million, is being sued in Delaware federal court by investors who say founder Benjamin Fulton misrepresented a marketing relationship between his firm and Barclays Plc. It caps an annus horribilis for Fulton in which a merger fell through, another adviser took over two of Elkhorn’s products, and the firm’s remaining funds shuttered. It’s a reminder that not everyone can make it in the fast-growing ETF industry.