Which was domesticated first, the horse or the dog?
Without doubt the dog by a wide margin. As pointed out in this thread, the horse is difficult for exact dating but archeological evidence has horse related burial presence back to circa 2,000 BC. The domestication of the dog variant from the wolf dates, at a minimum back to 18,000 BC with arguments of a date 20,000 years earlier.
The chicken. Without a chicken there'd be no egg.
Joe Biden