Notable Examples: The Beast (1975), Wedding Trough (Vase de Noces) (1975), Emanuelle in America (1977), Howard the Duck (1986)
No, bestiality porn does not count, as it would be far too easy to find examples. Instead, it's much more rewarding to find feature-length films screened for large audiences that featured humans with animals, especially Howard the Duck. Because that movie is batshit.
But that's jumping too far ahead in the timeline. First, 1975's The Beast, a French erotic comedy featuring a very explicit sex scene between a young maiden and a hairy, well hung beast.
In the same year, Wedding Trough debuted. It's subtitled "The Pig Fucking Movie." Enough said.
In 1977, the Italian film Emanuelle in America was released, featuring a controversial scene in which an actress gets intimate with a horse.
Thankfully, Howard the Duck saved the world from these unsavory scenes by depicting sweet love between a giant talking duck and Lea Thompson. Wait, what are we talking about? That scene was traumatizing.