Originally Posted by Philomel

I think they have the right to give consent, but they don't have the ability. After all, consent not given is consent denied -- having sex with someone who's, say, unconscious is still rape, even though they did not actively deny consent. As they do not have the ability to give consent (at least not to humans), it really doesn't even get into rights.

As for your question about Catherine the Great:

Source: snopes.com: Unbridled Lust

There have, however, been a fair few documented cases of injuries caused by sexual intercourse with horses.