1660s, "a horse whose natural gait is a pace," agent noun from pace (v.). As "one who measures by pacing," by 1835.
Entries linking to pacer
pace v.
1510s, "to walk at a steady rate," from pace (n.). Meaning "to measure by pacing" is from 1570s. That of "to set the pace for" (another) is from 1886. Related: Paced; pacing.