Cat (ぬこ, nuko) is a Japanese internet slang from Futaba Channel and is a somewhat shorten form of cat (にゃんこ, nyanko) in hiragana.
It was in 2004 when the 4-koma Cat Cafe, a minor manga known only to those in the know, was advertised on Futaba Channel, it started the term. Even though the manga tile was ねこきっさ, Futuba introduced as ぬこきっさ.
The word came to be used in some parts of Futaba Channel, and an original character Middle-aged Cat which originated from Futaba, was created.
It has been used by cat lovers from time to time since then, but be careful not to abuse it, as there are more or less people who are disgusted by this terminology.
In fact, this word is the name of an animal that has existed for a long time, and does not mean cat, but a type of fox called a cat fox (ぬこ).