Twice, actually.
1. After being married for several years, my ex-wife and I were talking about what kind of porn we watched, and she was interested in seeing my collection. I warned her that some of it was weird, like bestiality weird, and she wasn't turned off. We watched some of it together, and some time later she asked if I wanted to see her do that. I said I'd never ask it of her, but if she was actually into it I'd love to watch. She sort of shrugged and said she'd be willing to try. We never made it happen though, and ended up divorcing amicably for unrelated reasons.
2. Having had positive results the first time, I initiated a similar conversation with my next long-term partner much sooner. As it turned out, they were a closet zoo themself and had fantasized about fucking a dog. I got them an ElypseArt dog dildo, and we eventually got to make their fantasy a reality, with some help from an owner on this site. We may even get our own intact male someday (though we currently have too many pets already, lol).
It may be confirmation bias, but having had two consecutive partners who were both at least willing to try it tells me that it's probably way more common than most would believe. It's just not the kind of thing most would admit on a survey.