The boy was killed in the southern Philippines when he was yanked into the water in front of his horrified family.
"One fight there, another here—at the age of 16, I killed someone. A person, really," he said.
The president has said he will quit office if it is proven that any of his family were involved in drug trafficking.
Bertha the hippo had been living at the Manila Zoo in the Philippines since 1959.
It is not the first time the Philippines president has referenced cannibalism.
The attack by Islamist militants on Marawi had "all the hallmarks of a professional military operation."
ISIS claimed responsibility for the shooting via its Amaq news agency.
Philippine authorities say there's no evidence of terrorism in the attack which left 36 dead and dozens wounded.
Earlier, President Donald Trump said Washington was closely monitoring what he described as a "terrorist" attack unfolding in Manila.
Dozens of people are reported injured after an attack at a resort in Manila.
The Philippines' president has repeatedly shown hostility towards the U.S.
His war on drugs has descended into a bloody killing spree that defies the rule of law.
She could have booked the flight she was born on for 25,000.
Duterte gave no signs he would moderate his anti-democratic promises, like killing criminals without trial or enacting policies without the legislature's approval.