Who was the fastest horse ever clocked?

Officially, Secretariat set records in all three races of the Triple Crown in 1973, the year he won it. All three still stand today.

His records:

Kentucky Derby 1 and 1/4 miles 1:59 2/5

Preakness Stakes 1 and 3/16 miles 1:53

Belmont Stakes 1 and 1/2 miles 2:24

Dr Fager ran a second quarter in the Washington Park Handicap of :20 3/5, which is the fastest quarter mile ever in the body of a race.