Super Kitty was one of the many super wishes that Timmy wished up in the past. As a result of making many super wishes that soon became very dangerous, Timmy and his godparents made an agreement about making super wishes. Super Kitty only appeared in a flashback shown in "Super Bike".
When Timmy was about to wish for a super bike, Cosmo and Wanda stopped him before he made the wish. Cosmo reminded him of this wish when trying to explain why Timmy should not wish for a super bike. According to the flashback that was shown, Super Kitty was so big and vicious that it even scared the local dogs away. It then pounces on presumably Timmy since he was running away with the dogs. What happened next is unknown as the flashback ended at that point, but it can be assumed that Super Kitty was unwished to prevent any further damage. If so it would have been most likely sent to Unwish Island due to its violent nature.