Nyan Nyan Nyanko

Nyan Nyan Nyanko (にゃんにゃんにゃんこ)

First released in July 2000.

Small, usually white kittens that imitate various items, most commonly food. They also appear as everyday items, such as toys, school supplies, or household necessities.

The term "Nyan" or "nyaa" is a Japanese onomatopoeic word for a cat's meow. "Ko" is a word which in Japanese connotes with "child" or "young." Together they make the childish/cutesy word for kitten.

Source: wikipedia