It depends, for some is the kinky aspect, which means it's not a physical attraction, but the concept of having sex with an animal and what it means to them is what makes it attractive.
For some it is physical, which is equivalent to why normally people get attracted to other people, and in this aspect is not so easy to explain. One could say it's because at the end of the day, a penis is a penis (or vagina), you will just be attracted to it anyway; you may find it as/or more hot than a human penis/vagina.
But it's not just that, because some people are attracted to the animal itself and find an animal hot just as other people find a person hot. For this I would say we could compare it in some way if you think an animal is beautiful or not, regardless if you feel sexual attraction to it.
For me is a mixture of both kink and actual attraction to the animal. The concept of having sex with an animal turns me on, and I'm attracted to their sexual organs, I actually think they are hot. And for the animal itself, I do found them beautiful, but I don't see them 'hot' as I would with a person.
All of these are my opinions, I'm not a scientist or anything like that. It's just deduction and hypothesis. Hope it helps though.