Dr. Toxic

Dr. Toxic was a scientist working with Equestrian Disease Control (EDC) as a virologist. Working with numerous diseases, such as Severe Acute Equine Flu, Crimson Shock, Reaper Virus, Alicorn Fungal Syndrome, Agent 513, and numerous other viruses, bacteria, and funguses that are easily spread, and are highly lethal. After trying to release a bioweapon called Necro Plague, he was fortunately caught and sent to Tartarus as a traitor towards the EDC, Princesses, and all of Equestria. He was released by a mysterious unicorn called Anarchy, he said he wanted to form a team of pony villain's to bring down the Princesses, Mane 6, and plunge Equestria into darkness and chaos. Anarchy said that he could be the brains of the team, so now helps Anarchy to form a uncurbable virus called the Malleus Strain, that can even infect Alicorns. 

He always had a fascination towards diseases when he was a colt, so it was no surprise when he landed a job at the EDC. But at 6 years old, he was diagnosed with Multiple Personality Disorder after his father was sent to Tartarus because of treason, his father was responsible starting a revolt which was put down quickly, he could've started a civil war. After that, Dr. Toxic thoughts got more grim and his former friends saw him slowly going insane by the day. He also started to built up anger inside of him of the princesses for sending his father to Tartarus. So he was happy to join Anarchy to destroy Equestria.