Background: Free-range pig farming represents a minor proportion of pig production in France but is attracting an increasing number of farmers because of societal expectations and the opportunity to use pasture-grazed forage. However, this type of farming faces several challenges, including biosecurity, parasitic management, and contact with wild fauna and pathogenic flora. Case presentation: Two Gascon pigs raised on an outdoor fattening farm in the Hautes-Pyrenees department of France were submitted after sudden death for necropsy at the National Veterinary School of Toulouse. The pigs were of two different breeds but from the same group of 85 animals that had grazed on a 4-ha plot of land being used for grazing for the first time. Based on an in-depth interview with the farmer, the epidemiological information available, and the necropsy and histology examinations, a hypothesis of great eagle fern intoxication was proposed. Although the sample of animals available for diagnosis was small, the success of the administered therapy confirmed our diagnosis. It was recommended that in the short term, the animals be prevented access to the eagle fern by changing their pasture or removing the plants. Vitamin B1 and vitamin B6 were administered via feed as Ultra B® at 1 mL per 10 kg body weight per day for 2 days (providing 9 mg thiamine (vitamin B1) and 0.66 mg pyridoxine (vitamin B6) per kg body weight per day). Marked remission was observed, with 6 of 10 intoxicated animals with symptoms surviving (yielding a therapeutic success rate over 50%), but the therapy did not compensate for the loss of initial body condition. In total, of the 85 animals in the group after intoxication, 6 died, and 6 recovered. Conclusions: The significance of this report lies in the scarcity of eagle fern intoxication cases reported in the literature, though such intoxication may become a significant problem as the development of outdoor rearing continues. Thus, eagle fern intoxication should be included in the differential diagnosis of nervous system symptoms in swine. The case also emphasizes the importance of anamnesis and discussion with the farmer as an essential step to guide diagnosis.