Japan is a country which likes to adapt the cuisines brought into her. The most typical example is curry which is originally from India which was brought to Japan via England. Also there are many dishes which you can see in Japan but not in the original country.
Napolitan is definitely the one which you can not order in Naples, Italy. It is a pasta dish using spaghetti. The spaghetti is boiled and later pan fried with other ingredients such as onion, green pepper, bacon(bacon can be replaced by ham or sausage) and ketchup/tomato sauce. Onion, green pepper and bacon/ham/sausage have to be pan fried before adding the boiled spaghetti.
Napolitan can be found in Kissaten - Japanese style coffee shop and diners. Although it is an adapted Japanese pasta dish, it is hard to find at Italian restaurants even run by Japanese chefs. Anyway, if you have found Napolitan on the menu and you want to try something which you can eat only in Japan, Napolitan is a good choice.