Soma
Confidently identified in much popular writing, the plant's true identity actually has no scholarly consensus at all. Multiple named twentieth-century researchers have proposed entirely different candidates — a mushroom, a desert shrub, and at least two other separate plants — and every single identification has its own specific modern proponent rather than any ancient confirmation.
Rigveda, Mandala 9; R. Gordon Wasson's Amanita muscaria proposal (1968), among several competing later identifications
A ritually pressed sacred plant central to the ancient Indian Rigveda, whose actual botanical identity has been unknown since antiquity itself.