I would really appreciate some research or logic in the story. Calvary, especially heavy calvary, did not fight in heavy rains.
It made the ground soft during the charge which caused the horse to trip. This would pile up a charge and at the very least break the leg of the horse which led to either it being killed by the opposing force, or killed by their own forces to put it out of its misery.
A good cavalry mount was worth many men in terms of cost and training.
No calvary man would risk a charge in rain.