In 1986, a 52-pound orange and white cat named Fatso enjoyed wearing T-shirts and jamming on a keyboard piano. And although he used up the last of his nine mortal lives in the 1990s, Fatso snagged his 15 minutes of fame in 2009 after his owner, Charles Schmidt, uploaded a minute-long video to YouTube of Fatso grooving on the ivories. The video has attracted more than 16.2 million views to date. Now Schmidt is taking e-retailer Threadless.com to court for a T-shirt design purportedly birthed as a result of Fatso’s fame.

Two graphic designers enjoyed watching Fatso so much that they created a T-shirt design they called Three Keyboard Cat Moon, which shows three large kitties in T-shirts playing keyboards in the light of a full moon, and submitted it to Threadless.com, the T-shirt e-retailer that lets its fans create and vote on T-shirt designs. The full moon element came from the design of another T-shirt called Three Wolf Moon, which had gained popularity among consumers in 2009. It featured three wolves howling at the full moon and was sold through Amazon.com. It remains one of Amazon’s top-selling clothing items today.

But it is Three Keyboard Cat Moon T-shirt, which became a best-selling design for Threadless and still graces T-shirts and iPhone cases the e-retailer sells today, that is at the center of a catfight. Schmidt sued Threadless in U.S. District Court in Spokane, WA, last week, alleging copyright and trademark infringement, and is seeking an unspecified amount of damages from the Chicago-based e-retailer.

Threadless did not respond to a request for comment and has not responded publicly to the suit. However, included in the court filing submitted by the plaintiff is a Dec. 2, 2010, letter from Threadless’ attorneys addressed to Schmidt’s attorney. It says Schmidt has “significant hurdles to overcome in order to prove his allegations.” It says that the U.S. Copyright Office did not register Schmidt’s copyright until Sept. 22, 2010, more than a year after Threadless first produced the Three Keyboard Cat Moon tee, and that means Schmidt cannot recover damages.

The e-retailer also says Schmidt abandoned his copyright when, on his web site, he encouraged consumers to submit their own videos in homage to Fatso. A recent mash-up—where two content sources are merged to create something different—combine a Fatso look-alike with YouTube pop sensation Rebecca Black’s video and song “Friday,” for example.  “Requesting submissions from third parties who have used his work unquestionably conveys the message that “Cool Cat” is, quite literally, free for the taking to anyone who wants it,” Threadless’ attorneys wrote.

Schmidt’s filing asks the court to award him damages and profits Threadless earned from the design. It also asks for the destruction of all Three Keyboard Cat Moon merchandise.