The animated series:
- What Could Have Been: Two failed pilots were made by Total Television but were not shown on TV until Underdog went into syndication. One was "Cauliflower Cabbie" (a canine cabbie who became do-gooder The Champion), and "Gene Hattree" (a Quick Draw McGraw ripoff in that it starred a singing cowboy horse).
- The Wiki Rule: The Underdog Wiki.
- Acting for Two:
- Actor Allusion:
- California Doubling: The fictional Capitol City is mainly Providence, RI, and a few shots have Boston's skyline; the poster has New York City, for whatever reason.
- Dyeing for Your Art: Patrick Warburton bleached his dark brown hair platinum blonde for the role of Cad Lackey.
- Late Export for You: The film wasn't released in Japan until two years after its initial theatrical run.
- Real-Life Relative: In the Latin Spanish dub, Luis Daniel Ramírez voices Underdog and his nephew Diego Ángeles voices Jack.
- Voice-Only Cameo: Jess Harnell has a voice-only appearance as an astronaut who spots Underdog in space.