Agree with everyone above, it is so easy to cause problems with rugs. With a native pony that is not clipped, too light weight a rug flattens their wonderful winter coat, and they end up colder than they would have been without, and too heavy a rug causes sweating which can quickly make a pony uncomfortable or even ill. If you do choose to rug (and I do have 2 with rugs on just now, so I am not completely anti rug!) they require much more careful monitoring. It is almost easier once the weather gets worse and at least there is less of a worry about them getting too hot.
I would not be particularly worried about cold, persistent wet tends to have more of a negative effect.
If you do wish to get a rug to have one in case you feel you need one, Ruggles are very good, they do ones with and without neck covers, but the ones with neck covers are elasticated behind the ears so they don't blow back. They have an ebay shop. I just discovered them last year, and are very good quality for the price - I have always had more expensive brands in the past, but tried these as they had very good recommendations for being correct dimensions for small ponies, and I am very impressed. If you are buying one to have in just in case, I would go for 200gm - they are not going to end up colder with that than with no rug, but are less likely to overheat either.