Kion’s reaction to Twilight calling The Mark of the Guard a Cutie Mark in this image is more inline with his attitude in season 3 while under the effects of the cobra venom.

• Horses are prey animals, the first thing they usually do is to instinctively panic and run away when predator approaches.  
• Twilight panics and runs instinctively by prey animal instinct, she needs enough time to clear space and enough sanity to cast the spell to fight back her predator. Talk about having magic doesn’t always mean we’re invincible, mentality status is important.  
• This chase was just started, and Twilight haven’t think or prepared of casting any spells just yet.

It’s not on his behind.