Hamato Yoshi is a fictional character from the Teenage Mutant Ninja Turtles fictional universe. In all continuities, he was once a great and honourable ninja whose story is always closely intertwined with that of Splinter, but he is almost always no longer around during the course of the story.
In most versions he was the owner of a pet rat who became Splinter; this includes the original comics and Teenage Mutant Ninja Turtles 2003 cartoon series. The earlier 1987 cartoon series, the Teenage Mutant Ninja Turtles Adventures comics, and the 2012 cartoon series provided a twist in that Hamato Yoshi was Splinter, having been turned into a giant rat.
Read more about Hamato Yoshi: Comics, 1987 Cartoon & TMNT Adventures, Movies, 2003 Cartoon, 2012 Cartoon