Obscurial

A young witch or wizard, usually under the age of ten but sometimes older, who has kept their magical ability repressed until it becomes an Obscurus – a violent force of Dark Magic which may burst forth from them unexpectedly (WFT).

• In the 1920s, Newt Scamander encountered an eight-year-old Obscurial in Sudan and tried to save her by removing the Obscurus from her body, but she unfortunately died anyway. A few months later when Percival Graves found the leftover Obscurus inside Newt's suitcase and accused him of using it to kill people and destroy buildings in New York City. Scamander explained that the Obscurus once removed from the Obscurial's body has no magical power and cannot hurt anyone (WFT).
• Credence Barebone, adopted child of Second Salem anti-magic activist Mary Lou Barebone, was the oldest known Obscurial as he reached young adulthood. The Obscurus force inside him was responsible for the deaths of Senator Shaw and Credence's mother Mary Lou and sister Chastity. His Obscurus went on a rampage through New York City while trying to kill Percival Graves/Grindelwald, who had hoped to use his power against the No-Maj. It was destroyed by Aurors under orders from MACUSA President Seraphina Picquery, although whether this was the end of Credence as a wizard is unknown (WFT).

Harry Potter never became an Obscurial because even though the Dursleys disapproved of magic, they did not completely stop him from using his powers all the time. In a FAQ on her new Official Site, J.K. Rowling wrote: "Even when he was scolded for ‘making things happen’, he didn’t make any attempt to suppress his true nature, nor did he ever imagine that he had the power to do so" (JKR).