Since I'm usually watching mating videos and not zoo porn, I'd have to say I'm more interested in seeing the animal having a good time, than in the person involved. Although I watch human porn as well, I still kinda dislike stuff that looks more acted out than two people having fun with each other. I guess that's what kinda ruins porn for me in general. In many cases it looks way too staged. Especially in professional porn, but also in amateurish stuff.
My favorite human porn is solo masturbation stuff, where the focus lies on the enjoyment of the person in front of the camera. That's something that turns me on, the enjoyment of the sex. It's something I just can't feel if people are going through repetitive motions, perform mechanical looking thrusts without passion, while making the obligatory moans or while they groan like being in pain.
When it comes to animals involved, I have similar desires. I want to see the animals enjoying themselves, but most often than not it's not the case. The animal might just not be interested and either being forced or coaxed to having sex. I'm absolutely against forcing anyone to have sex, be it human or animal, but even coaxing isn't really my thing. I guess that happens in most filmed sexual encounters between people and animals and I don't mind coaxing animals into it, as long as they have a choice and can leave if they want to, but that doesn't necessarily make an entertaining piece of media.
Paradoxically I kinda enjoy watching casual/boredom sex between people, despite my previous statement, as watching one party letting the other one having fun, without personally really caring either way can be quite arousing for me, the same doesn't apply to sex between people and animals however.
If the animal hasn't the initiative and gets aroused by the interaction, then it looks to me rather bored and therefore not interested, which ends my fun immediately. I've seen way to many sexploitation videos, where sex-actors pose with animals or rub their naked body against the animal while the animal looks rather indifferent. Those videos, in my eyes, turn animals into props, rather than passionate lovers. And this is, why I usually care more about the animal having fun, rather than the person involved, because it's easy for the person to have fun, it's way harder for the animal.