Phlogistan
Phlogiston was a substance hypothesized to embody heat energy.
It was not an unreasonable hypothesis; it stood until people found seemingly more precise explanations. But we still don’t really UNDERSTAND energy, we only have sets of equations to explain how it behaves. For all we know we could be all soaked with “phlogiston” but we’d never know it from experiments because it doesn’t act like anything we know.