I've actually had this happen with other than an NPC Sim. I was playing three different households in this particular game. When I rejoined the household whose toddler had aged to a child that day while I played a different household, the child aged to teen when I took back control of his household. It was like that day was his birthday and the game recognized him aged up to child already and when I actually took control it was still his birthday, so he aged up with me able to watch it. He became the eldest in the family and I completely missed all of his childhood. It sounds like something similar happened with the horse. Maybe it had just aged up from a foal that day, then when you got control of it, it aged up again.