There's no way Long, with his incredibly weak and stupid troops, would have actually managed to capture those strong, evolved zombies from the streets. He must have figured out a way to keep feeding regular zombies humans and encourage weird mutations/development paths. That would explain why he kept arranging the games, which looked like pure nonsense. They actually served a purpose. All that being said, Long is a real, honest-to-god psycho mad scientist for trying to purposefully create stronger zombie, when he's nothing but a weak, unevolved human himself.