Brian J. O'Connor

FILE — Kix, a pit bull mix, stands in his kennel at the ASPCA Animal Hospital in New York on Sept. 24, 2015. The ProShares Pet Care E.T.F. invests in an index of companies involved with health care for pets, including pharmaceuticals, specialty retailers and distributors. (Brian Harkin/The New York Times) XNYT187 XNYT187

There is a fund for everything. But why?

From cannabis to the gig economy and even the Cow exchange-traded note, which bets on livestock, these niche exchange-traded fund are proliferating as fund managers...