Distance covered by mules and donkeys counts towards distance covered by horses.
Steps to reproduce:
Spawn a mule and/or donkey
Put a saddle on the mule and/or donkey
Ride the mule and/or donkey and walk forwards
Observed Result:
In the escape menu -> statistic -> general tab, scroll down to "Distance Covered by Horse". It increases.
Expected Result:
The "Distance Covered by Horse" statistic should NOT increase since you haven't rode a horse.
Camels are already being tracked in the report you originally mentioned, therefore there is no reason to track them in this report too. I have removed any mention of camels from your report, and added 'Donkeys' in place of it, as you did not mention that mob.