In the OP's context, a "Zebra" is an obscure illness, disease or condition affecting a comparatively small number of people, but with symptoms somewhat similar to one or more different and more common illnesses. It is so named because these diseases/conditions are sometimes misdiagnosed by well-meaning (but often inexperienced) doctors when something far more common is actually the cause. Medical students are told "remember, if you hear hoof beats, think horses, not zebras".
Doctor 1: "I think this patient may have Castleman's disease."
Doctor 2: "Castleman's disease is a zebra, there are far more likely causes of his fever and weight loss."
The fictional characters of the TV show "House" were known for "chasing zebras".
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