As I understand it horses get 3/4 of their warmth from the forage they eat (internal radiator), and growing too fast is much more harmful to a youngsters joints than at a slow steady rate. So while rugging might speed growth, I would still say dont rug unless the horse is showing signs of being uncomfortable or it for eg you have weeks of rain (if that happens and they dont have anywhere to dry off they might be prone to developing rain scald). The other thing to bear in mind is that however safely fitted, rugs can present a hazard and while all horses should be regularly checked anyway, a youngster with a rug on might need checking more often than is practical - wee monkeys they are! But do start to get them used to wearing a rug, having feet picked up etc while they are youngish so it doesnt become an issue later.