A 4 year old Friesan mare was referred to the Cornell Equine Hospital in apparent heart failure. The week prior to admission, the mare had become inappetent and febrile, was observed to cough, and had a markedly bounded pulse. The mare was treated presumptively for Potomac horse fever and respiratory disease. Re-examination by the referring veterinarian revealed pronounced pallor, tachyarrhythmia, and malformed manure. On presentation, the mare was severely dehydrated with marked pallor. A tachyarrhythmia was ausculted. She had pronounced jugular groove pulses bilaterally, a bounding facial artery pulse, and cool extremities. She was tachypneic with a variable respiratory rate, but clear lung sounds. She had pitting edema in the ventrum. An EKG revealed a supraventricular tachycardia with atrial bigemini. Ultrasound showed clear lungs with no pericardial fluid. Bloodwork showed a hyperlactatatemia, and elevated cardiac Troponin I. Initially, myocarditis was prioritized. However, a cardiac consultation revealed no apparent cardiogenic cause for her shock. This elevated a suspicion of vasculitis or pathologic mass effect obstructing venous return to the heart. The mare responded adversely to volume resuscitation, developing worsening ventral edema, bicavitary effusion, and hypotension. Due to her decline, she was euthanized and remains were submitted for necropsy. This case highlights the clinical signs, diagnostics, treatment, and differential diagnoses of equine heart failure. A special focus is placed on vascular leakage in the absence of primary cardiac disease. Syndromes unique to the Friesan breed are also discussed.