Any halters we have, even the small adjustable foal halters, are too large for the foals until they are a few months old. That's not a problem, as I make a harness which I think, work much better, and are far safer.
I do not like having my foals pulled and tugged by those delicate little necks and faces, so I make these harnesses, which work like a charm. Our foals go easily from the harness to the halter, when they are larger, and I haven't had one problem. (YET!)
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I make the chest and girth strap wider, so that it equalizes the pressure if they pull, and have a small "carry handle" on the top, with the lead ring, so that I can actually lift them up a bit, if they get too silly.
: it's hard to run away when your feet don't have any traction. :lol:
This picture gives you an idea...tho' it's one you will have to squint at.