Propagators appear in waves of one or more during the "Propagator" mutator in Co-op Missions. It was introduced in the "Futile Resistance" mutation, and uses the model of a changeling, albeit larger and with void energies surrounding it.
Propagators will appear on the minimap as an exclamation mark in a circle. If it comes close to an allied unit, it will instantly kill the unit or structure and turn them into a copy of themselves, regardless of how powerful the unit is or whether they are ground or air. Even summoned spawns and heroic units are affected.
Propagators should be neutralized from range. They can be affected by crowd control effects (e.g. Concussive Shells from marauders, Kerrigan's Immobilization Wave, etc.), giving the players time to eliminate them before they come close.