There's two things here that need separating. One is you are human and therefore have thoughts. Some will be pretty weird. But we all think. The second is that these thoughts are bothering you so much they are making your life very difficult. This second part is part of an anxiety disorder, and if you're not currently in therapy for it you need to start.
So, thoughts are thoughts and they can pop in our head and it doesn't mean you want to REALLY have sex with a dog or a bear. Being attracted to the taboo can be just a random thing here and there. I wouldn't read too much into it. If you are plagued by it though and it starts to become too 'real' could you talk to a professional about it? Obviously we don't want you doing anything to an animal. But I sincerely doubt that when it came down to it, you would. If you 'think' you would, please talk to a psychologist.
How are other things in your life? Sometimes we are kind of bored, lacking in some area, feeling lost or a deficit in some way and then our mind wonders. Then we fixate and it's a cycle. So, how are other things in your life?