Chris Windholz reached into Keystone Lake west of Tulsa Sept. 24 to grab what he thought was a drum, a common Oklahoma fish.
"I picked it up and thought, 'that's one goofy looking drum,'" he told the Tulsa World.
Windholz and his friend, Chance Ward, 16, decided the fish, about 12 inches long, was some kind of piranha.
"He opened the mouth and it had a full set of teeth," Windholz said. "I'm glad I didn't stick my thumb in there when I picked it up."
However, searching the Internet convinced Windholz the fish was probably a plant-eating pacu.
Chris Whisenhunt, a fisheries biologist with the Oklahoma Department of Wildlife Conservation, said pictures he's seen of Windholz's fish lead him to agree, and that in his 10 years with the department he has seen several.
"It's actually quite common," he said. "It's not like there have been hundreds but there have been several."
Pet owners usually dump them, he said.
"They grow quite large and can get bigger than people can take care of so they end up getting dumped somewhere."