Um is this a pokemon mod?
Anyways if it is Follow these steps
(Horse Script)
#1 Ride the Pokemon
#2 Open inventory or press E on your keyboard
#3 Take the saddle and put it in the saddle Slot
(Pig Script)
#1 Take the saddle and press the build button on the pokemon
#2 Then hit build button again on it
#3 You shoud be riding it
(If none of these work)
Ask the modpack developer how to put a saddle on the pokemon.
Thanks for asking!
-NightmareGameee