i dont think you are wrong, disgusting or gross if you are just having thoughts like this. the very fact that you are questioning it shows that a part of you is ambivalent about it anyway. a lot of ppl have sexual thoughts that they would never act upon, but those thoughts can cause them a lot of distress. if this applies to you, i would seek out a therapist to gain an understanding of how to just accept those thoughts and let them go without beating yourself up about it.

if this, on the other hand, is reflecting a desire... again, i'd say that you don't need to act on desires. i don't think bestiality is right or morally permissible. if you have engaged in something like that before, i would encourage you to stop and seek out support so you don't do it again.

i think that a lot of shame can revolve around issues like this (all sexual matters, really) but i know a lot of therapists are competent in being able to help people in a non-judgemental manner. you won't be vilified for having these thoughts or for having acted on them in the past, but i doubt very much (and hope!) that no one here will respond in a way that encourages you to act on them/continue acting on them.