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A 25-year-old pony mare was presented to the clinic with prelimina- rily reported severe acute colic. The pony during the previous week had shown inappetence, apathy and fever of unknown origin. Clinical examination and placement of a gastric tube were indicative of a secondary gastric dilation. Rectal exploration found moderate caecal meteorism with a tensed and painful medial taenia as well as a dilated and fluid-filled small intestine. In addition, a solid, mobile, non-painful structure of approximately 10 cm diameter was palpated ventrally. A hyperechogenic mass close to the caecum was detected using ultrasonography of the abdomen in the right flank. The peritoneal fluid was an exudate with cytological signs of an acute to subacute peritonitis. Blood analysis showed markedly increased plasma lactate concentration and a marginal neutrophilia and lymphopenia, with a total leukocyte count of 6 G/l. Because the owners refused consent for a laparotomy and the pony showed increased signs of severe pain despite conservative medical treatment, it was euthanized. The main findings on necropsy were extensive adherence of the caecal apex to the right and left colon and the ileum as well as a neoplasia in the lumen of the caecal apex. The mass, which was covered with a mucous membrane, had a tough consistency. The cut surface was grey-white to beige-coloured and multilobular with numerous necrotic and acute haemorrhagic areas. According to histopathological and immunohistochemical findings, the mass was characterized as a gastrointestinal stromal tumour (GIST), a rare neoplasia in horses.