(part of common anime words)
Well, nekomimi 猫耳 is actually two words joined together. The first one, neko 猫, as you might have guessed, means "cat," one of the words for animals in Japanese. The other, mimi 耳, means "ear," it's a word for the many body parts of the body. Together they, uh, form "cat ears," or sort of, which is one of the many attributes of cat girls in anime.
Nekomimi for catsThough nekomimi does mean "cat ears" in Japanese, cats do not have nekomimi. Because that'd be redundant. You'd end up saying neko no nekomimi 猫の猫耳, "cat ears of the cat," and as we can see we got neko twice there. It's obvious, that "the ears of the cat," neko no mimi 猫の耳, would be "cat ears," nekomimi, so that word is only used for people who have or wear cat ears, never for the cats themselves.
Difference between Nekomimi and Cat GirlsAnother thing is that cat girls usually have more than just cat ears nekomimi. They have, for example, a tail, shippo 尻尾, and, sometimes, a "collar" too, kubiwa 首輪, and often say nyaa にゃあ. So the nekomimi themselves are nothing more than one of the many attributes a complete "cat girl" would have.
Nekomimi and Cat BoysLike I literally just explained, nekomimi is a word for an attribute and doesn't really mean "cat girl," so you can use the word nekomimi for "cat boys" too if you're a fan of Loveless or whatever. If you want to distinguish between genders, just use nekomimi as part of the predicate before a gendered noun.
- nekomimi joshi 猫耳女子
nekomimi musume 猫耳娘
- nekomimi danshi 猫耳男子
- nekomimi-tsuki joshi 猫耳付き少女
Girl wearing cat ears.
- nekomimi-tsuki danshi 猫耳付き少年
Boy wearing cat ears.
Cat Girl and Cat Boy in Japanese
That said, you can still say "cat girl" and "cat boy" in Japanese. Duh, obviously! Using the same principle as above, just use a gendered noun with the word "cat," neko 猫, instead of nekomimi.
- nekobito 猫人
- neko joshi 猫女子
neko musume 猫娘
- neko danshi 猫男子
Other Animal Ears (Not Cat Ears)
Of course you can say inu mimi 犬耳, "dog ears," nezumi mimi 鼠耳, "mouse ears," and so on for whatever animal you want, that is, as long as you know the name of the animal. (here's a list) If you want a generic term, "beast ears," kemono mimi 獣耳, would be your best bet.