Bit ambiguous, but probably the Canton Bulldogs (who were formed in 1905, possibly as a semi-pro team, but went pro in the formation of the NFL in 1920).
However, a large number of other teams were formed or went pro in 1920 who might be possible contenders include:
- Akron Pros / Indians (1920-26)
- Buffalo All Americans / Bisons / Rangers (1920-29)
- Chicago Tigers (1920)
- Cleveland Tigers (1920-21)
- Columbus Panhandles / Tigers (1920-26)
- Dayton Triangles (1920-29)
- Detroit Heralds (1920)
- Hammond Pros (1920-26)
- Muncie Flyers (1920-21)
- Rochester Jeffersons (1920-25)
- Rock Island Independents (1920-25)
Probably the best rival claim is the Akron Pros, who were at least a semi-pro team as early as 1908.
