They say now horses can be microchipped in the same way vets work with pets, branding is no longer required.
Paul King from People 4 Ponies said: "It is just barbaric in this day and age for human beings to be doing such terrible things."
Hot branding is banned in the UK for cows and sheep, but is still allowed for ponies.
However others say microchips aren't as good as branding because you need to be really close to an animal to scan the chip, and the horses don't stand still for long enough when people go near them.
Although the horses are wild and live on the moor, farmers look after them to help them when they're sick.
They also make sure that the correct numbers of male and female ponies live in the wild so the herds continue to grow in a controlled way.
Farmers say the branded numbers help them locate a sick or injured animal if a member of the public spots one. At the moment microchipping technology isn't good enough to identify an animal from a distance.
Hot branding isn't the only way to mark animals - some people think another method called freeze branding may be better because it causes less physical pain.
Freeze branding takes longer than hot branding, and for wild animals any contact with people can be distressing, so that may not be a perfect solution either.