Sounds rather extreme to me, something better left at realms of the story section. If you are "fiendishly creative" there are without a doubt many more punishments to teach your sub to obey that won't involve animals.
Then again, I read your saying your sub in fact wanted to eplore that taboo. Makes me wonder if this indeed was a punishment for her?

Don't want to judge peoples fetishes, but as far as I am concerned, I have no experience and no wishes in bestiality. For me, a punishment likt that would be a reason to leave. But... to each his own.