Mari Lwyd is not pre-Christian or ancient.
My personal take is that as the industrial age began and the old feudal custom of feting employees with a party and liquor died out, someone (a clever, thirsty poet, obviously) resurrected an Old Gray Mare and a contest that used the language of the working class of Wales (the landowners and bosses would not be as fluent) to get that beer.https://museum.wales/articles/1187/Christmas-Traditions-The-Mari-Lwyd/
"With the earliest account of the Mari dating from 1798, the boom years, as regards to the amount of horse's heads in existence, were between 1850 and 1920."
Image from Wikipedia
