9) The flies.
8) The smell.
7) Possible exposure to whatever killed the horse.
6) Your arm will get tired.
5) Takes time/attention away from the living horses.
4) Unlike a piñata, there is no prize inside -- just more dead horse.
3) Does nothing to solve the problem of having a dead horse lying around.
2) You look like an idiot.
1) The horse is dead.
Today's lesson: Don't beat a dead horse
Next: Hopefully something for Towel Day