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Nightmare Fuel: Street Sharks
Every. Single. Transformation. The transformation sequences are very painful to watch, since the transforming person is nearly always screaming in pain as his face and body change in a disturbing way. What happened to the Bolton brothers is really creepy.
To specify what happened to them: First, they find out that their dad has gone missing. For all they know, he's been killed. Immediately after, they're kidnapped by two horrific humanoid fish mutants and taken to a lab, where they are given injections of something or another, which seemingly kills them. They wake up in a sewer drain and, after a brief cooldown period, painfully turn into humanoid fish mutants themselves.