

Epona is a very likable horse, but I don’t believe that she will be able to win in this battle. Nami has her Weather staff that can summon various elements and these would deal a lot of damage to Epona. Epona doesn’t really have a way to escape from Nami’s attacks and she will eventually be defeated. Nami is very athletic to she would be able to dodge any incoming charges from Epona. Nami wins.
On Luffy’s ship after this battle:
Sanji: (in a mood of joy and excitement, stentorian) WHO WANTS HORSE MEAT FOR DINNER!?
Crew (Luffy included): ME, ME, ME, ME, ME, ME, ME!
Poor Epona though, she was always a fun horse