Time each horse in each race. Rank each horse according to its time and then you have them all ranked.
If, for example you have the five fastest horses in the first race and they are timed, they could all be faster than the horses in the following races, so why would you drop any of them off in favour of slower horses.
Equally, you could have the five slowest horses in the last race and why should a slow horse be given preference over a faster horse in the first race.
