Because one can legally consent and the other one can’t.

This is not fucking rocket science, people. 

well, technically speaking, the pony CAN give you consent and that particular one IS of age and they ARE a sentient species in their land so your argument falls apart. Most of them ARE of age, CAN consent and are NOT technically animals so if you WERE able to get to Equestria, it is possible that you can have sex with a pony… Of course, the only one you would have a chance with is Lyra.

Only on tumblr would one discuss about fucking ponies.