Hound Town (partially lost NBC animated TV show pilot; 1989)
Hound Town was a 1989 animated pilot created for NBC by Prudence Fraser with the intention of catering to a prime-time audience. The pilot was rejected, however.
It was directed by Ralph Bakshi at his Bakshi Animation studio, where he worked with animators from Mighty Mouse: The New Adventures, such as Rich Moore, Andrew Stanton and Jeff Pidgeon. Bakshi claims this to be one of his least favorite works. It has (allegedly) only aired once.
The pilot was available on Youtube at one point, but the channel has since been suspended, meaning it's still lost and only a few screenshots of it remain.
UPDATE: As of 8/30/2016, the pilot has been uploaded to MEGA. The copy originates from MySpleen, and is notable for its poor quality and lack of the first few minutes.
UPDATE: As of 9/3/2016, the pilot has also been uploaded to Youtube by Lost Media Wiki contributor RSTVideo.