As Splinter tells the story, many, many years ago there were two dueling forces: one of positivity, and one of negativity. Positivity prevailed and banished the negative one into what later became Mount Shasta.
Years later, the negativity, Kurko, escaped and for some reason ended up in the Sewers of New York City. There he fought and defeated the Ninja Turtles. After causing an earthquake in New York, he retreated back to Mount Shasta.
Kurko's plan back on his "home" turf was to destroy the balance of Yin and Yang. Unfortunately for Kurko, the Turtles allied with them and were transformed by Yang into the giant mutant turtle, El Gran Terrapín. After a lengthy battle, El Gran Terrapín beat Kurko and stuffed him back into the mountain.