Tuna is great in pasta, so it makes sense for pizza. The oddest thing I've seen on pizza is raw eggs ... if you break the yolks and spread them out while the pizza is hot, they sort of cook in front of you. Had that in Switzerland.
I was actually just explaining this to a friend yesterday. When you say 'Peperoni' in Germany, people think of paprika and all its spicy variants. You'll have to order a 'salami' pizza if you want the meat. Germans also have a weird thing for pizza with tuna (Thunfischpizza). I know it sounds gross, but I would highly recommend trying it if you haven't already. It's kind of like a tuna melt!
In Italy, the generic word for the sausage things is salami. A shop that sells them is a salumeria. I've never seen a sausage labeled as 'pepperoni' there. (But my experience is all in the northern half of Italy ... I can't rule out different food terms in the south.)
It's also peppers in Italy. My family was once in a pizzeria in Padova when a young American guy, maybe 20, ordered pepperoni pizza for himself and a young woman. Then we heard him complaining "where is the pepperoni on this pizza", which was very confusing to the waiter who could see the pizza covered in pepperoni.