He can definitely calm the horse, but he won't be able to send it back given that it's currently trapped. However, with enough persuasion the horse will lead Emet-Selch to the site of the altar and will dig at the ground with its hooves, as if trying to tear up the remains of the ritual marking. However, when it tries there will be black and red flames that fly up and burn the horse, causing it aggravate and even seem to empower the horse to make it grow.