I have just brought a NF forest yearling even though he is not my first youngster he is 6 months younger than Copper was when he first came and Copper was very well handled and brave and this poor thing is still a little bit of a scaredy cat.
On to the question what is better a leather Foal slip or a nylon one? He will be wearing the headcollar in the field during the day but will have it taken off at night once he has settled.
I would go for leather if you have to have on on him as leather will break easier. Personally though, I would be hesitant about turning him out with one in case he becomes caught up and it doesn't break.
A friend of mine lost her foal due to getting caught up with a leather slip on, it didn't break, foal struggled and broke it's neck. I never turn out with head collers on now.
"Horses - be to their virtues ever kind, and to their faults a little blind"
if i turn out with a foal slip on (i rarely do ) i use a field safe one there nylon but the loops that hold it together are made to pull apart under pressure . in my exsperience leather does nor brake on a foal as the strength and weight of a foal dosent seem to have enough force to brake it .