Nyo is a word, or sound effect, commonly used with cats. Other forms of "Nyo" are "Nya" and "Nyu". The main character of DiGi Charat, Dejiko, who happens to dress as a catgirl, uses "Nyo" to end all of her sentences; Her partner, Puchiko, a smaller version of a feline female chooses to end her sentences with "Nyu".
"It's the laughing ghost-Nyo!"
"Dejiko, what are you smoking-Nyu?"