I don't think Ralphie lit the fire. I think Ralph's outburst about it being just a horse was because Ralphie's son was in the HOSPITAL hooked up to wires and hoses like a vegetable. Ralphie was DESTROYED by it. He even confessed to a priest, and started that fund [or trust?]. Ralphie just didn't care about the horse and was mad that Tony seemed to be showing more of a care to the horse situation then his son's, hence the way he talked to Tony about it being just a horse and him eating beef all the time.