When the Colombian drug lord Pablo Escobar was killed in 1993, most of the animals he had imported as pets — zebras, giraffes, kangaroos and rhinoceroses — died or were transferred to zoos. But not his four hippopotamuses. They thrived. Perhaps a little too well. Officials estimate that about 170 hippos, descended from Mr. Escobar’s original herd, now roam Colombia, and the population could grow to 1,000 by 2035, posing a serious threat to the country’s ecosystem.