Some propellants have wildly different melting points. The most extreme case is the theoretically fantastic (and nightmarish in practice) Fluorine-Lithium-Hydrogen tripropellant, with liquid hydrogen being very cryogenic while lithium melts at 180°C. So instead of having cryogenic, room-temperature or even heated fuel tanks, have there ever been rocket designs where a liquid propellant is stored in solid form?
This would include melting the propellant with a heat exchanger before pumping it into the chamber, storing it as a powder and moving that instead, or even storing it in a suspension of particles in a liquid. Slush hydrogen and similar is outside of the scope of this question, though.
Note that this isn't about hybrid rockets. In hybrid rockets, a liquid propellant is running against a solid grain, with the reaction happening at the interface. The reaction itself may melt the solid grain, but that's at best a way to improve the local reaction, it isn't expected to flow anywhere. This is instead about liquid-fuelled rockets, where some of the fuel is stored as a solid, but at some point moved around like a liquid.