VelocityVectorProperty

Example:

//Create an entity with a billboard rotated to match its velocity.
const position = new Cesium.SampledProperty();
position.addSamples(...);
const entity = viewer.entities.add({
  position : position,
  billboard : {
    image : 'image.png',
    alignedAxis : new Cesium.VelocityVectorProperty(position, true) // alignedAxis must be a unit vector
  }
}));