The shortest horse race under rules in the UK is 5 furlongs.
That’s 0.625% of a mile. The fastest it has ever been run is 53.69 seconds - an average speed of 41.91 mph.
In Australia 800 metre races are common - less than 4 furlongs. The quickest this distance has been covered is 42.7 seconds. That represents an exact equivalent of the mean speed for 5 furlongs.
To get even shorter, one needs to go to the United States. One furlong races are commonplace (and may be seen live on certain UK bookmakers' screens - providing one stays up late enough). The winner is usually rewarded with $5000 - more than for many UK races.
Naturally, someone had to go further (or, perhaps, less far). Here is a film of ‘The Shortest Horse Race in the World’ - 8 seconds of action from Louisiana Downs...
Half a furlong is covered.
Effectively a 100 metre sprint for equines. It is of anecdotal interest to note that had Usain Bolt lined up beside them, he would have led early (horses take a while to get going) and been beaten only by about 15 yards.