If you are a fan of turtles in general this is your shirt.
For some reason the turtles have inspired great fictional characters from the very Ninja Turtles to the gigantic Gamera. going through iconic villains like Bowser.
Regarding the ninja turtles, it all started as a joke by Kevin Eastman and Peter Laird who made a black and white comic book, cruelly gloomy and rather gore in 1984. They wanted to pay homage to Ronin and Frank Miller's Daredevil, in fact the turtles and Daredevil shared origin. The unexpected success, sold so many copies that a comic of a single edition became a series of comic strips.
Regarding Gamera, they say that the creation of gigantic mutant monsters responds to Japanese society's fear of nuclear tests being turned against them. So both gamera and godzilla respond to a feeling of guilt of the Japanese people.