Add horses_spook, in the list of triggers for the mission template you want.
Code:
(
"lead_charge",mtf_battle_mode,charge,
"You lead your men to battle.",
[
(1,mtef_defenders|mtef_team_0,0,aif_start_alarmed,12,[]),
(0,mtef_defenders|mtef_team_0,0,aif_start_alarmed,0,[]),
(4,mtef_attackers|mtef_team_1,0,aif_start_alarmed,12,[]),
(4,mtef_attackers|mtef_team_1,0,aif_start_alarmed,0,[]),
],
[
horse_spook,
...