Now we are in late July and some of the ponies have started their annual, limited period of eating small amounts of the fronds of bracken. Though poisonous it is something that Exmoor ponies tend to do at this time of year.
Thankfully they usually avoid this plant for most of the year. Bracken is toxic because it contains large amounts of thiaminase, which breaks down thiamine which is an important B vitamin. That said, a pony has to consume pretty large amounts of bracken before poisoning will occur. It has to consume 3-5% of its body weight for at least a month to induce symptoms. Bracken is also considered carcinogenic.