> Mass–energy equivalence states that all objects having mass, or massive objects, have a corresponding intrinsic energy, even when they are stationary. In the rest frame of an object, where by definition it is motionless and so has no momentum, the mass and energy are equal or they differ only by a constant factor, the speed of light squared (c²).
Now go check the "History" section of that article; which, by the way, takes up about half of it.
I'm trying to come up with other examples, but as the parent said, it then depends how pedantic we want to be e.g. take Poincaré's spacetime, but he'd still be working from two pre-existing ideas, "space" and "time"; the idea of combining the two is (feels?) quite novel and unexpected.
Going back some more, the notion of "space" (or that of "time") feels more primitive, less explainable in terms of other notions.