The best replacement for underdog is overcat.

The best alternatives to underdog are rated as following:
★★★★★  overcat
★★★★  mighty mouse
★★★★  krypto
★★  dark horse
★★  hong kong phooey
★  underduck
★  topdog
★  favorite nfl
★  rockzap
  superdog
  underdog
  favorite

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Speaking about underdog, one probably shouldn't forget about fear lyrics.

Other search terms related to underdog and their ratings are:
★★★★★ fear lyrics
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underdog
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mighty mouse