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:
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The first National Football League teams to play against each other were the Allegheny Athletic Association and the Pittsburgh Athletic Association. The first game was played in 1892.?æ