Usagi had appeared in the Teenage Mutant Ninja Turtles' TV series in the '80s/'90s, beginning with an episode aptly titled "Usagi Yojimbo."
He has been making guest appearance in the current animated series, introduced in the 4-part Battle Nexus storyline. Leonardo visits Usagi's world in another story, and many of my secondary characters are introduced. Usagi and Gen the rhino bounty hunter even make it to New York in another episode. Needless to say, these stories are not part of my comic book continuity.
Usagi and Gen will be showing up in the coming season as well. Here are a couple of character designs: