rival (n.)

1570s, from Latin rivalis "a rival, adversary in love; neighbor," originally, "of the same brook," from rivus "brook" (from PIE root *rei- "to run, flow"). "One who is in pursuit of the same object as another." The sense evolution seems to be based on the competitiveness of neighbors: "one who uses the same stream," or "one on the opposite side of the stream" A secondary sense in Latin and sometimes in English was "associate, companion in duty," from the notion of "one having a common right or privilege with another." As an adjective 1580s from the noun.

c. 1600, from rival (n.). Related: Rivaled; rivaling.