The forerunner of the Christmas pudding (aka. plum/figgy pudding), a rich fruit pudding, was called Frumenty, it was served in Medieval times. Frumenty was a spiced porridge, It has its origins in a Celtic legend of harvest god, Dagda, who stirred a porridge made up of all the good things of the earth.
The pudding became specifically associated with Christmas, rather than merely any festive occasion, when it was introduced to the Royal Christmas dinner table by Prince Albert.