Allereun gets to react first (Crit on Observe Roll against their Stealth Roll). 

Fiore failed his Riding Roll to keep Odysseus under control. When riding a non-war trained horse into battle first thing a Riding Roll (DL 25) is required to keep them under control or the rider cannot do anything for the first round. 

Failure by 10 or more means the horse runs off, with the rider on it. 

A Fumble and the horse throws you off, leaving you Prone, runs off. 

Fiore needs to get his horse war trained ( or get a War horse), which is what Animal Handling is for (for training animals)