Mainly Thoroughbreds are thought of as racehorses but other horses are used like Arabs (not as fast as thoroughbreds and don't jump fences or hurdles), Akhal-tekes (speed is not comparable to thoroughbreds but is famously heat- and drought- resistant with high stamina) and Quarter Horses (the oldest all-American breed who were originally raced over quarter-mile stretches).
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Secretariat was able to reach a top speed of 49 mph.
This is not really a question that can be answered. Different races have different distances and are run over different types of tracks (grass or dirt) under different conditions. Technically you consider the speed of a horse by how fast he runs a race not the speed at which he runs which varies over the course of the race. There are too many variables that make this question impossible to answer.
There are different records for each race so it is possible to find what horse ran di. For example the record for the fastest Kentucky Derby ever ran was set by Secretariat in 1973 and is 1 min 59 secs 40 hundredths.
Some of the fastest horses that ever raced are Man O' War and Secretariat are considered to be two of the fastest racehorses of all time.
If you mean which breed then the Thoroughbred is the 'king' of the race track. Coming in second would be Quarter horses and Arabians.