He is a character of the kids program of Japanese TV company Fuji TV. However, since he is the character produced on April 2, 1973, he is loved by not only a child but the adult. A lot of his fancy goods is sold now, too.
ガチャピン&ムック モコモコ ムック
ガチャピン・ムック 武将 根付 ストラップ ( 足軽 )
By the way, he has a fellow called Gachapin. He has already used blog and Twitter, and both are very popular. Gachapin utilized social media until now than Mukku. Can Mukku beat him?