difficult.
As people said, the first problem is your definition: What counts as that? Someone who did it once? Did it once because he wanted it, or because he was horny and this was the only option? Someone who can get off to the porn? Someone who lives exclusively with an animal as a life partner?
Depending on that, the percentage is going to vary wildy. 'Being able' to get off (at least once) on horse/women porn is going to be 50% of the populace, very coarsely. That's going to be the largest possible percentage. Exclusively living together and having sex with an animal as a "married" partner is probably going to be 0.0001 to 0.00001% of the populace. That is coarsely going to be the smallest possible percentage or order of magnitude thereof.
Those are my estimates. Depending on your definition, pick between that. But you likely should stick to the very small numbers.
A few observations: In general, the "zoo" population seems to be extremely skewed to the young generations. The cohorts appear distributed - to me - like Zipfs law, with those aged 16 to 18 making up a majority. And pretty much all male. After that, the "dropout" rate is really high.
This indicates - kill me - that for most this is an option they stray to to avoid women but they are still horny. So what to do? Be it that they are too shy, socially awkward and similar. The older they become, the more they get over their shyness and similar and settle in "more normal" relationships.
Just something to consider in the "what is zoo" definition.
Another thing: Kinsey is quite outdated, and the "accessibility" to farm animals has gone down a lot since then. You know adolescent boys, don't you? If the parents have a few cows on the family farm, you just know they'll stick it in there, because teens are horny as hell and every hole is a goal.
The percentage of people who live and work on farms is almost negligible nowadays. In Kinseys time, it was still a large portion of the populace.
This could be worked with both ways: Maybe we would need to correct a nowadays-number up. Nowadays teens/people are still born zoos, but don't have the means to 'live like that' as they are all stuck in cities. OR: maybe we need to correct the old Kinsey number down, cause fewer people were "real zoos", but the opportunities were so plentyful, even "non-zoos" did it.
To support both paragraphs: People do "settle" with things or poke around not knowing what they really like, don't they? You can see it microscopically here when zoos report they like horses but live with dogs - not everyone has the money for a horse, so second option it is.
And you can see it with the masses of "secretly-gay" people who have entire families because "that was expected of them".
Just for perspective: Here
Demographics of sexual orientation - Wikipedia is a collection of mostly self-identification studies, and there 1% to 10% of the populace says they are "fully" gay/lesbian.
There is no way in my mind that there is more than a tenth as many zoos as there are gays/lesbians. Far less. Because otherwise where the fuck is everyone? Running around in public in a "free" country you will run into someone who pings "gay" eventually. If there were as many zoos, where are they? Riding stables would look a lot different, considering that due to the need to keep horses there, they all needed to bunch up at those locations.