Not all ponies are the same. A German riding pony has completely different needs than a Shetland pony, to name two extreme examples. The type of husbandry, the state of health, the age and the season also play a role. A healthy pony of a robust breed would usually only need a rain cover for longer stays outdoors during the summer.
Most pony breeds develop a dense winter coat, so that they only need a pony rug in an open stable or for transport in the trailer. As a rule, a filling of 50 g is sufficient for a pony rug. If your pony is old or ill, you should consult a vet before buying a rug.