Cat Boy

In anime, a cat boy is a character that has cat-like features, such as feline body parts—"cat ears," nekomimi 猫耳, "cat eyes," nekome 猫目, "cat mouth," nekoguchi 猫口, and a "cat tail," neko-shippo 猫しっぽ—and ending their sentences in nyaa にゃー, "meow."

Character: Ferris フェリス
Anime: Re:Zero kara Hajimeru Isekai Seikatsu Ｒｅ：ゼロから始める異世界生活 (Episode 12)

• In Japanese
• Tropes

In Japanese

The most common way to say "cat boy" in Japanese would be nekomimi-danshi 猫耳男子, "cat-ears boy." In general, if a character has cat ears they probably have a cat tail, too, but that's not necessarily true.

You can combine either neko 猫, "cat," or nekomimi 猫耳, "cat-ears," with a noun that means "boy" to create a word that means "cat boy" or "cat-ears boy." Unfortunately, there are various words that mean "boy" in Japanese. For reference:

• nekomimi-shounen
  猫耳少年
  Cat-ears boy.
  • shounen
    少年
    Underage person. (legal term.)
    Boy. (normal usage.)
• nekomimi-otokonoko
  猫耳男の子
  Cat-ears boy.
• otokonoko
  男の子
  Male child.
  Boy.
• nekomimi-otokonoko
  猫耳男の娘
  Cat-ears trap.
• otokonoko
  男の娘
  Male girl. A girl that's actually a boy. A boy that looks like a girl. A trap.
• nekomimi-seinen
  猫耳青年
  Cat-ears boy.
  • seinen
    青年
    A young adult man.
• nekomimi-shota
  猫耳ショタ
  Cat-ears boy.
  • shota
    ショタ
    Boy anime character. (male counterpart of loli.)

Similarly: neko-danshi 猫男子, neko-otokonoko 猫男の子, neko-shounen 猫少年, neko-seinen 猫青年, neko-shota 猫ショタ, and so on.

The same thing also works if you switch "boy" for "man."

• neko-otoko
  猫男
  Cat man.
• nekomimi-otoko
  猫耳男
  Cat-ears man.

Tropes

See: Cat Girl for common things and terms related to cat boys.

It's worth noting that neko ネコ is a LGBT slang for "bottom" in a homosexual relationship.