Tetsumi Onamota is an anthropomorphic ram and the first new character created for Teenage Mutant Ninja Turtles: Amazing Adventures.
Tetsumi was once like a brother to Hamato Yoshi and Oroku Saki. As a child, he discovered a key that was tied to the Japanese Zodiac. Using this key, he harvested the life force and powers of animals who featured in the zodiac. This power began to corrupt him, and it cost the lives of half of the Hamato Clan. Yoshi and Saki fought and defeated him, thought by both boys to have been killed. However, he later appeared in the present, transformed into a ram man. He hypnotized Chris Bradford, Dr. Cluckingsworth, Tiger Claw, Anton Zeck, Hamato Miwa, Leatherhead, Dr. Tyler Rockwell, and Splinter to following him. He used the key to drain their powers and transformed himself further.
Shredder and the Turtles arrive, each on their own accords, to fight and defeat Onamota and his new minions. During the fight, Donatello recognizes the Key as being Kraang technology. Donatello and then Raphael each in turn lost the Key, but it was recovered by Splinter. Splinter broke the Key, seemingly destroying his old friend. However, Splinter reasons that since he had thought Onamota dead before, and that the Zodiac power made him unpredictable, there was a chance that he was still alive. As he does, Onamota's signature yin and yang symbol is seen on the moon.