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"Animal" just means something that lives in the sense that it moves, it's "animated".
By this definition, there's nothing inherently wrong with fucking animals. Determining whether fucking an animal is wrong or not should be done on a per-species and per-gender basis.
Humans are animals, it's fine to fuck humans.
Dogs are animals, it's not fine to rape a small female dog, as it can be hurt, but there's nothing possibly wrong in getting fucked by a male dog, as it can't possibly be hurt in any conceivable way.
Some Pokemon have above-human intellect, so they should be the ones to decide if it's fine for us to fuck them. But they still are sentient, able to consent, and are able to clearly communicate unambiguously their needs.