I dont know exactly what you're asking, but i'll answer for the two possible things
To make the hero MOUNT when an item is used, on the DB in the "items" tab make the item non consumable (or consumable, it's your choice) only used from menu and make it call a common event
on the common event make it runs automatically and change the hero graphic to the mounted one
that's it for the common event.
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To make the game CHECK if the player is mounted and DISMOUNT him when he uses the SAME item
make the same common event from before but put a IF condition, if the "mount switch" is on, change the graphic to the walking hero and put the "mount switch" to OFF
else, change the graphic to the mounted hero and put the MOUNT switch to ON
this way, everytime you want to check if the hero is mounted just check if the switch is on