Hey now, the ancient Celtic kings used to fuck a horse to represent their virility and domination over the land. To think on it, it was more of a horsey donkey-punch situation.

I don't have a link to the particular documentary, but it's in the first episode, hosted by anthropologist Richard Rudgely.

Sexy Beasts Looks back to a time before sex was taboo, when humans saw themselves as an integral part of the natural world. Through history and prehistory, the representations of the ancient gods and traditions followed by pagans have been marred by propaganda from other religious groups eager to rein in those they defined as 'wild barbarians'. In truth, the word pagan is a Roman term meaning 'country folk', and the general concept of paganism is of oneness with nature and a quest to fully understand the world around us. Though historical accounts lead us to images of stone dildo-wielding women flashing their genitals at cattle, chieftains having sex with horses before slaughtering them and whipping sessions in mixed saunas, the underlying theme is of human similarity with animals and nature. Where modern religion aims to emphasise the difference between humans and the world around us, portraying sexuality as taboo, the ancient pagan perspective blurs these boundaries and explores all the elements of the world on equal terms. http://docuwiki.net/index.php?title=Pagans_%28Ch4%29