It's funny - I kill these guys all the time, and get the message "my standing with Epona has decreasaed", then I go in and turn on the N64 and put in Legend of Zelda: Ocarina of Time, and Epona still likes me just fine... :)
It says that the Epona Faction is reduced. Should I be worried about this when going into the frontiers? I can just imagine running accross a Named horse Epona that will stomp on me as I run to Midguard...