Complex sentence: Prior to 2006, the horse was running to its left with its head turned to its right which is a mistake in heraldry because all "beasts" must face dexter (to their right or the viewer's left).
Split into simpler sentences: Prior to 2006, the horse was running to its left with its head turned to its right. In theory this is a mistake in heraldry because all "beasts" must face dexter (to their right or the viewer's left).