When petsmounts are healed, they do not gain XP. Healing items do not count as food either, so you always need to feed your pets after healing them (if you haven't fed them yet). You can feed them multiple healing items, until they're max health. Petsmounts can be healed by feeding them:
A petsmount needs to be fed at least once every 72 hours. If 3 days pass without feeding, if you give it unsuited food or if you lose a battle, your petsmount loses 1 HP.
Customizing your Destruction Dragosteed
Destruction Dragosteed Harnesses: